In the visual formatting model, each element in the document tree
generates zero or more boxes according to the box
model. The layout of these boxes is governed by:
The visual formatting model does not specify all aspects of
formatting (e.g., it does not specify a letter-spacing algorithm). Conforming user agents may behave
differently for those formatting issues not covered by this
specification.
User agents for continuous media
generally offer users a viewport (a window or other
viewing area on the screen) through which users consult a
document. User agents may change the document's layout when the
viewport is resized (see the initial containing block).
When the viewport is smaller than the area of the canvas on which
the document is rendered, the user agent should offer a scrolling
mechanism.
There is at most
one viewport per canvas, but user
agents may render to more than one canvas (i.e., provide different
views of the same document).
In CSS 2.1, many box positions and sizes are calculated with respect
to the edges
of a rectangular box called a containing block. In
general, generated boxes act as containing blocks for descendant
boxes; we say that a box "establishes" the containing block for its
descendants. The phrase "a box's containing block" means "the
containing block in which the box lives," not the one it generates.
Each box is given a position with respect to its containing block,
but it is not confined by this containing block; it may overflow.
The details of
how a containing block's dimensions are calculated are described in
the next chapter.
The following sections describe the types of boxes that may be
generated in CSS 2.1. A box's type affects, in part, its behavior in the
visual formatting model. The 'display' property, described below,
specifies a box's type.
Block-level elements are those elements of the source document that are formatted visually as
blocks (e.g., paragraphs). Several values of the 'display' property make an element
block-level: 'block', 'list-item', and 'run-in' (part of the
time; see run-in boxes), and 'table'.
Block-level elements (except for display 'table' elements, which are described in a later chapter) generate a principal
block box that contains either only block
boxes or only inline boxes. The principal block box establishes the containing block for descendant boxes and
generated content and is also the box involved in any positioning
scheme. Principal block boxes participate in a block formatting context.
Some block-level elements generate additional boxes outside of the
principal box: 'list-item' elements. These additional boxes are
placed with respect to the principal box.
(and assuming the DIV and the P both have 'display: block'), the
DIV appears to have both inline content and block content. To make it
easier to define the formatting, we assume that there is an anonymous block box
around "Some text".
Diagram showing the
three boxes, of which one is anonymous, for the example above.
In other words: if a block box (such as that generated for the DIV
above) has another block box or run-in box inside it (such as the P
above), then we force it to have only block boxes and run-in
boxes inside it.
When an inline box contains a block box, the inline box (and its inline ancestors within the same line box) are broken around the block. The line boxes before the break and after the break are enclosed in anonymous boxes, and the block box becomes a sibling of those anonymous boxes.
Example(s):
This model would apply in the following example if the following
rules:
body { display: inline }
p { display: block }
were used with this HTML document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HEAD>
<TITLE>Anonymous text interrupted by a block</TITLE>
</HEAD>
<BODY>
This is anonymous text before the P.
<P>This is the content of P.</P>
This is anonymous text after the P.
</BODY>
The BODY element contains a chunk (C1) of anonymous text followed
by a block-level element followed by another chunk (C2) of anonymous
text. The resulting boxes would be an anonymous block box around the BODY,
containing an anonymous block box around C1, the P block box, and
another anonymous block box around C2.
The properties of anonymous boxes are inherited from the
enclosing non-anonymous box (e.g. in the example just below the subsection heading "Anonymous block boxes", the one for DIV).
Non-inherited properties have their initial value. For example,
the font of the anonymous box is inherited from the DIV, but the
margins will be 0.
Properties set on elements that cause anonymous block boxes to be
generated still apply to the boxes and content of that element. For
example, if a border had been set on the BODY element in the above
example, the border would be drawn around C1 (open at the end of the
line) and C2 (open at the start of the line).
Some user agents have implemented borders on inlines containing
blocks in other ways, e.g. by wrapping such nested blocks inside
"anonymous line boxes" and thus drawing inline borders around such
boxes. As CSS1 and CSS2 did not define this behavior, CSS1-only and
CSS2-only user agents may implement this alternative model and still
claim conformance to this part of CSS2.1. This does not apply to UAs
developed after this specification was released.
Inline-level
elements are those elements of the source document that
do not form new blocks of content; the content is distributed in lines
(e.g., emphasized pieces of text
within a paragraph, inline images,
etc.). Several values of the 'display' property make an element
inline: 'inline', 'inline-table',
and 'run-in' (part of the time; see run-in boxes).
Inline-level elements generate inline
boxes.
The <p> generates a block box, with several inline boxes inside
it. The box for "emphasized" is an inline box generated by an inline
element (<em>), but the other boxes ("Some" and "text") are inline boxes generated by a block-level element (<p>). The latter are called anonymous inline
boxes, because they don't have an associated inline-level element.
Such anonymous inline boxes inherit inheritable properties from
their block parent box. Non-inherited properties have their initial
value. In the example, the color of the anonymous inline boxes is
inherited from the P, but the background is transparent.
Whitespace content that would subsequently be collapsed away according to the 'white-space' property does not generate any anonymous inline boxes.
If it is clear from the context which type of anonymous box is
meant, both anonymous inline boxes and anonymous block boxes are
simply called anonymous boxes in this specification.
There are more types of anonymous boxes that arise when formatting
tables.
If the run-in box contains a block
box, the run-in box becomes a block box.
If a sibling block
box (that does not float and is not
absolutely positioned) follows the run-in
box, the run-in box becomes the first inline box of the block box.
A run-in cannot run in to a block that already starts with a
run-in or that itself is a run-in.
Otherwise, the run-in box becomes a block box.
A 'run-in' box is useful for run-in headers, as
in this example:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>A run-in box example</TITLE>
<STYLE type="text/css">
H3 { display: run-in }
</STYLE>
</HEAD>
<BODY>
<H3>A run-in heading.</H3>
<P>And a paragraph of text that
follows it.
</BODY>
</HTML>
This example might be formatted as:
A run-in heading. And a
paragraph of text that
follows it.
Despite appearing visually part of the following block box,
a run-in element still inherits properties from its parent
in the source tree.
Please consult the section on
generated content
for information about how run-in boxes interact with generated
content.
This value causes an element to generate a block box, which itself
is flowed as a single inline box, similar to a replaced element.
The inside of an inline-block is formatted as a block box, and the
element itself is formatted as an inline replaced element.
This value causes an element (e.g., LI in HTML) to generate a
principal block box and a list-item inline box. For information about
lists and examples of list formatting, please consult the section on
lists.
none
This
value causes an element to generate no boxes in the formatting structure (i.e.,
the element has no effect on layout). Descendant elements do not
generate any boxes either; this behavior cannot be
overridden by setting the 'display' property on the descendants.
Please note that a display of 'none' does not create an invisible
box; it creates no box at all. CSS includes mechanisms that enable an
element to generate boxes in the formatting structure that affect
formatting but are not visible themselves. Please consult the section
on visibility for details.
This value creates either block or
inline boxes, depending on context.
Properties apply to run-in boxes based on their
final status (inline-level or block-level).
Floats. In the float model,
a box is first laid out according to the normal flow, then
taken out of the flow and shifted
to the left or right as far as possible. Content may
flow along the side of a float.
Absolute positioning.
In the absolute positioning model, a box is removed from
the normal flow entirely (it has no impact on later siblings)
and assigned a position with respect to a containing block.
Note.
CSS 2.1's positioning schemes help authors make their documents
more accessible by allowing them to avoid mark-up tricks
(e.g., invisible images) used for layout effects.
The box's position is calculated according to the normal flow (this is called the position in
normal flow). Then the box is offset relative to its normal position. When
a box B is relatively positioned, the position of the following box is
calculated as though B were not offset. The effect of 'position:relative' on table-row-group, table-header-group, table-footer-group, table-row, table-column-group, table-column, table-cell, and table-caption elements is undefined.
absolute
The box's position (and possibly size) is specified
with the 'top',
'right',
'bottom', and
'left'
properties.
These properties specify offsets with respect to the box's
containing block. Absolutely
positioned boxes are taken out of the normal flow. This means
they have no impact on the layout of later siblings. Also,
though absolutely positioned
boxes have margins, they
do not collapse
with any other margins.
fixed
The box's position is calculated according to the 'absolute'
model, but in addition, the box is fixed with respect to some reference.
As with the 'absolute' model, the box's margins do not collapse with any other margins.
In the case of handheld, projection, screen, tty, and tv media types,
the box is fixed with respect to the viewport
and doesn't move when
scrolled. In the case of the print media type, the box is rendered on every page, and is fixed with respect to the page box, even if the page is seen through a viewport
(in the case of a print-preview, for example). For other media
types, the presentation is undefined.
Authors may wish to specify 'fixed' in a
media-dependent way. For instance, an author may want a box to remain
at the top of the viewport on the screen, but
not at the top of each printed page. The two specifications may be
separated by using an @media
rule, as in:
UAs must not paginate the content of fixed boxes. Note that UAs may print invisible content in other
ways. See "Content outside the
page box" in chapter 13.
User agents may treat position as 'static' on the root element.
An element is said to be positioned
if its 'position' property has
a value other than 'static'. Positioned elements generate
positioned boxes, laid out according to four properties:
for 'position:relative', see section
Relative
Positioning. For 'position:static', 'auto'. Otherwise: if
specified as a length, the corresponding absolute length; if
specified as a percentage, the specified value; otherwise, 'auto'.
This property specifies how far an absolutely positioned box's top
margin edge is offset below the top edge of the box's containing block. For relatively
positioned boxes, the offset is with respect to the top edges of the
box itself (i.e., the box is given a position in the normal flow, then
offset from that position according to these properties).
Note: For absolutely positioned elements whose containing block is based on a block-level element, this property is an offset from the padding edge of that element.
for 'position:relative', see section
Relative
Positioning. For 'position:static', 'auto'. Otherwise: if
specified as a length, the corresponding absolute length; if
specified as a percentage, the specified value; otherwise,
'auto'.
Like 'top', but specifies how far a box's right margin edge is
offset to the left of the right edge of the box's containing block. For relatively
positioned boxes, the offset is with respect to the right edge of the
box itself.
Note: For absolutely positioned elements whose containing block is based on a block-level element, this property is an offset from the padding edge of that element.
for 'position:relative', see section
Relative
Positioning. For 'position:static', 'auto'. Otherwise: if
specified as a length, the corresponding absolute length; if
specified as a percentage, the specified value; otherwise, 'auto'.
Like 'top', but specifies how far a box's bottom margin edge is
offset above the bottom of the box's containing block. For relatively
positioned boxes, the offset is with respect to the bottom edge of the
box itself.
Note: For absolutely positioned elements whose containing block is based on a block-level element, this property is an offset from the padding edge of that element.
for 'position:relative', see section
Relative
Positioning. For 'position:static', 'auto'. Otherwise: if
specified as a length, the corresponding absolute length; if
specified as a percentage, the specified value; otherwise,
'auto'.
Like 'top', but specifies how far a box's left margin edge is
offset to the right of the left edge of the box's containing block. For relatively
positioned boxes, the offset is with respect to the left edge of the
box itself.
Note: For absolutely positioned elements whose containing block is based on a block-level element, this property is an offset from the padding edge of that element.
The values for the four properties have the following meanings:
The offset is a percentage of the containing block's width (for 'left' or 'right') or height (for 'top' and 'bottom'). Negative values are allowed.
auto
For non-replaced elements, the effect of this value
depends on which of related properties have the value 'auto' as
well. See the sections on the
width
and height
of absolutely positioned,
non-replaced elements for details. For replaced elements, the
effect of this value depends only on the intrinsic dimensions of the
replaced content. See the sections on the width and height of absolutely
positioned, replaced elements for details.
Floats, absolutely positioned elements, inline-blocks, table-cells, and elements with 'overflow' other than 'visible' establish new block formatting contexts.
In a block formatting context, boxes are laid out one after the
other, vertically, beginning at the top of a containing block. The
vertical distance between two sibling boxes is determined by the 'margin' properties. Vertical margins
between adjacent block boxes in a block formatting context collapse.
In a block formatting context, each box's left outer edge touches the
left edge of the containing block (for right-to-left formatting, right
edges touch). This is true even in the presence of floats (although a
box's line boxes may shrink due to the floats), unless the box
establishes a new block formatting context (in which case the box itself
may become narrower due to the floats).
For information about page breaks in paged media, please consult
the section on allowed
page breaks.
In an inline formatting context, boxes are laid out horizontally,
one after the other, beginning at the top of a containing
block. Horizontal margins, borders, and padding are respected between
these boxes. The boxes may be aligned vertically in different ways: their
bottoms or tops may be aligned, or the baselines of text within them
may be aligned. The rectangular area that contains the boxes that form
a line is called a line box.
The width of a line box is determined by a containing block and the presence of floats.
The height of a line
box is determined by the rules given in the section on line height calculations.
A line box is always tall enough for all of the boxes it contains.
However, it may be taller than the tallest box it contains
(if, for example, boxes are aligned so that baselines line up).
When the height of a box B is less than the height of the line box containing it,
the vertical alignment of B within the line box is determined by
the 'vertical-align' property.
When several inline boxes cannot fit horizontally within a single
line box, they are distributed among two or more vertically-stacked
line boxes. Thus, a paragraph is a vertical stack of line boxes. Line
boxes are stacked with no vertical separation and they never overlap.
In general, the left edge of a line box touches the left edge
of its containing block and the right edge touches the right edge of
its containing block. However, floating boxes may come between the
containing block edge and the line box edge. Thus, although line
boxes in the same inline formatting context generally have the same
width (that of the containing block), they may vary in width if
available horizontal space is reduced due to floats. Line boxes in the same inline formatting
context generally vary in height (e.g., one line might contain a tall
image while the others contain only text).
When the total width of the inline boxes on a line is less than the
width of the line box containing them, their horizontal distribution
within the line box is determined by the 'text-align' property. If that
property has the value 'justify', the user agent may stretch the
inline boxes as well.
When an inline box exceeds the width of a line box, it is split into several boxes and these boxes are distributed across several line boxes. If an inline box cannot be split (e.g. if the inline box contains a single character, or language specific word breaking rules disallow a break within the inline box, or if the inline box is affected by a white-space value of nowrap or pre), then the inline box overflows the line box.
When an inline box is split, margins,
borders, and padding have no visual effect where the split occurs (or
at any split, when there are several).
Here is an example of inline box construction. The following paragraph
(created by the HTML block-level element P) contains anonymous text
interspersed with the elements EM and STRONG:
The P element generates a block box that contains five inline
boxes, three of which are anonymous:
Anonymous: "Several"
EM: "emphasized words"
Anonymous: "appear"
STRONG: "in this"
Anonymous: "sentence, dear."
To format the paragraph, the user agent flows the five boxes into
line boxes. In this example, the box generated for the P element
establishes the containing block for the line boxes. If the containing
block is sufficiently wide, all the inline boxes will fit into a
single line box:
Several emphasized words appear in this sentence, dear.
If not, the inline boxes will be split up and distributed across
several line boxes. The previous paragraph might be split as follows:
Several emphasized words appear
in this sentence, dear.
or like this:
Several emphasizedwords appear in this
sentence, dear.
In the previous example, the EM box was split into two EM boxes
(call them "split1" and "split2"). Margins, borders,
padding, or text decorations have no visible effect after split1 or
before split2.
Consider the following example:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Example of inline flow on several lines</TITLE>
<STYLE type="text/css">
EM {
padding: 2px;
margin: 1em;
border-width: medium;
border-style: dashed;
line-height: 2.4em;
}
</STYLE>
</HEAD>
<BODY>
<P>Several <EM>emphasized words</EM> appear here.</P>
</BODY>
</HTML>
Depending on the width of the P, the boxes may be distributed as
follows:
The margin is inserted before "emphasized" and after "words".
The padding is inserted before, above, and below
"emphasized" and after, above, and below "words". A
dashed border is rendered on three sides in each case.
Once a box has been laid out according to the normal flow or floated, it may be shifted relative to
this position. This is called relative positioning. Offsetting a box
(B1) in this way has no effect on the box (B2) that follows: B2 is
given a position as if B1 were not offset and B2 is not re-positioned
after B1's offset is applied. This implies that relative positioning
may cause boxes to overlap.
However, if relative positioning causes an 'overflow:auto' box to have
overflow, the UA must allow the user to access this content, which,
through the creation of scrollbars, may affect layout.
A relatively positioned box keeps its normal flow size, including
line breaks and the space originally reserved for it. The section on
containing blocks explains when a
relatively positioned box establishes a new containing block.
For relatively positioned elements, 'left' and 'right' move the
box(es) horizontally, without changing their size. 'left' moves the
boxes to the right, and 'right' moves them to the left. Since boxes
are not split or stretched as a result of 'left' or 'right', the
computed values are always: left = -right.
If both 'left' and 'right' are 'auto' (their initial values), the
computed values are '0' (i.e., the boxes stay in their original
position).
If 'left' is 'auto', its computed value is minus the value of 'right'
(i.e., the boxes move to the left by the value of 'right').
If 'right' is specified as 'auto', its computed value is minus the
value of 'left'.
If neither 'left' nor 'right' is 'auto', the position is
over-constrained, and one of them has to be ignored. If the 'direction' property of the containing block is 'ltr, the value of 'left' wins and 'right'
becomes -'left'. If 'direction' of the containing block is 'rtl', 'right' wins and 'left' is ignored.
Example(s):
Example. The following three rules are equivalent:
The 'top' and 'bottom' properties move relatively positioned
element(s) up or down without changing their size. 'top' moves the
boxes down, and 'bottom' moves them up. Since boxes
are not split or stretched as a result of 'top' or 'bottom', the
computed values are always: top = -bottom.
If both are 'auto', their computed values are both '0'. If one of them is
'auto', it becomes the negative of the other. If neither is 'auto',
'bottom' is ignored (i.e., the computed value of 'bottom' will be
minus the value of 'top').
Note.
Dynamic movement of relatively positioned boxes can produce
animation effects in scripting environments (see also the 'visibility' property).
Although relative positioning may be used as a form of superscripting and
subscripting, the line height is not automatically adjusted to take the
positioning into consideration. See the description of line height calculations for more
information.
A float is a box that is shifted to the left or right on the
current line. The most interesting characteristic of a float (or
"floated" or "floating" box) is that content may flow along its side
(or be prohibited from doing so by the 'clear' property). Content flows down
the right side of a left-floated box and down the left side of a
right-floated box. The following is an introduction to float
positioning and content flow; the exact rules governing float behavior are given in
the description of the 'float'
property.
A floated box is
shifted to the left or right until its outer edge touches the
containing block edge or the outer edge of another float.
If there's a line box, the top of the floated box is aligned with
the top of the current line box.
If there isn't enough horizontal room for the
float, it is shifted downward until either it fits or there are no
more floats present.
Since a float is not in the flow, non-positioned block boxes
created before and after the float box flow vertically as if the float
didn't exist. However, line boxes created next to the float are
shortened to make room for margin box of the float. If a shortened line box
is too small to contain any further content, then it is shifted downward until
either it fits or there are no more floats present.
Any content in the current line before a floated box is reflowed
in the first available line on the other side of the float.
In other words, if inline boxes are placed
on the line before a left float is encountered that fits in the remaining line box space,
the left float is placed on that line, aligned with the top of the line box, and then the inline boxes already on the line are moved accordingly to the right of the float (the right being the other side of the left float) and vice versa for rtl and right floats.
The margin box of a table, a block-level replaced element, or an element in the normal flow that establishes a new block formatting context
(such as an element with 'overflow' other than 'visible')
must not overlap any floats in the same block formatting context as
the element itself. If necessary, implementations should clear the
said element by placing it below any preceding floats, but may place
it adjacent to such floats if there is sufficient space.
Example(s):
Example.
In the following document fragment, the containing block is too narrow
to contain the content next to the float,
so the content gets moved to below the floats
where it is aligned in the line box according to the text-align
property.
Several floats may be adjacent, and this model also applies to
adjacent floats in the same line.
Example(s):
The following rule floats all IMG boxes with
class="icon" to the left (and
sets the left margin to '0'):
img.icon {
float: left;
margin-left: 0;
}
Consider the following HTML source and style sheet:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Float example</TITLE>
<STYLE type="text/css">
IMG { float: left }
BODY, P, IMG { margin: 2em }
</STYLE>
</HEAD>
<BODY>
<P><IMG src=img.png alt="This image will illustrate floats">
Some sample text that has no other...
</BODY>
</HTML>
The IMG box is floated to the left. The content that follows is
formatted to the right of the float, starting on the same line as the
float. The line boxes to the right of the float are shortened due to
the float's presence, but resume their "normal" width (that of the
containing block established by the P element) after the float. This
document might be formatted as:
Formatting would have been exactly the same if the document had
been:
<BODY>
<P>Some sample text
<IMG src=img.png alt="This image will illustrate floats">
that has no other...
</BODY>
because the content to the left of the float is displaced by
the float and reflowed down its right side.
As stated in section 8.3.1,
the margins of floating boxes never collapse with margins of
adjacent boxes. Thus, in the previous example, vertical margins do not
collapse between the P box
and the floated IMG box.
The contents of floats are stacked as if floats generated new
stacking contexts, except that any elements that actually create new
stacking contexts take part in the float's parent's stacking context.
A float can overlap other boxes in the normal flow (e.g., when a
normal flow box next to a float has negative margins). When this
happens, floats are rendered in front of non-positioned in-flow
blocks, but behind in-flow inlines.
Example(s):
Here is another illustration, showing what happens when a float
overlaps borders of elements in the normal flow.
Both paragraphs have set 'clear: left', which
causes the second paragraph to be "pushed down" to a position below
the float — "clearance" is added above its top margin to
accomplish this (see the 'clear' property).
This property specifies whether a box should float to the left,
right, or not at all. It may be set for any element, but only applies to
elements that generate boxes that are not
absolutely positioned.
The values of this property have
the following meanings:
left
The element generates a block box that is
floated to the left. Content flows on the right side of the box,
starting at the top (subject to the 'clear' property).
right
Similar to 'left', except the box is floated to the right, and content flows on the left side of the box, starting at the top.
none
The box is not floated.
User agents may treat float as 'none' on the root element.
The left outer edge of a
left-floating box may not be to the left of the left edge of its containing block. An
analogous rule holds for right-floating elements.
If the current box is left-floating, and there are any left-floating
boxes generated by elements earlier in the source document,
then for each such earlier box, either the left outer edge of the current box must be
to the right of the right outer edge
of the earlier box, or its top must be lower than the bottom of the
earlier box. Analogous rules hold for right-floating boxes.
The right outer edge of a
left-floating box may not be to the right of the left outer edge of any right-floating
box that is to the right of it. Analogous rules hold for
right-floating elements.
A floating box's outer top
may not be higher than the top of its containing block.
When the float occurs between two collapsing margins, the
float is positioned as if it had an otherwise empty anonymous block parent taking part in
the flow. The position of such a parent is defined by the rules in the section on margin
collapsing.
The outer top of a floating box
may not be higher than the outer top of any block or floated box generated by an element
earlier in the source document.
The outer top of an element's
floating box may not be higher than the top of any line-box containing a box
generated by an element earlier in the source document.
A left-floating box that has another left-floating box to its left
may not have its right outer edge to the right of its containing
block's right edge. (Loosely: a left float may not stick out at the
right edge, unless it is already as far to the left as possible.) An
analogous rule holds for right-floating elements.
A floating box must be placed as high as possible.
A left-floating box must be put as far to the left as
possible, a right-floating box as far to the right as possible. A
higher position is preferred over one that is further to the
left/right.
References to other elements in these rules refer only to other elements in the same block formatting context as the float.
This property indicates which sides of an element's box(es) may
not be adjacent to an earlier floating box. The 'clear'
property does not consider floats inside the element itself or in
other block formatting
contexts.
For
run-in boxes,
this property applies to the final block box to which the run-in box belongs.
Clearance is introduced as spacing above the margin-top of an element. It is used to push the element vertically (typically downward), past the float.
Values have the following meanings when applied to non-floating
block boxes:
left
The clearance of the generated box is set to the amount necessary to place the
top border edge below the bottom outer edge of any left-floating
boxes that resulted from elements earlier in the source document.
right
The clearance of the generated box is set to the amount necessary to place the
top border edge below the bottom outer edge of any right-floating
boxes that resulted from elements earlier in the source document.
both
The clearance of the generated box is set to the amount necessary to place the
top border edge below the bottom outer edge of any right-floating
and left-floating boxes that resulted from elements earlier in the
source document.
none
No constraint on the box's position with respect to floats.
Computing the clearance of an element on which 'clear' is set is done by first determining the hypothetical position of the element's top border edge within its parent block. This position is determined after the top margin of the element has been collapsed with previous adjacent margins (including the top margin of the parent block).
If the element's top border edge has not passed the relevant floats, then its clearance is set to the amount necessary to place the border edge of the block even with the bottom outer edge of the lowest float that must be cleared.
When the property is set on floating elements, it results in a
modification of the rules for
positioning the float. An extra constraint (#10) is added:
The top outer edge
of the float must be below the bottom outer
edge of all earlier left-floating boxes (in the case of 'clear:
left'), or all earlier right-floating boxes (in the case of 'clear:
right'), or both ('clear: both').
Note.
This property applied to all elements in CSS1. Implementations
may therefore have supported this property on all elements. In CSS2
and CSS 2.1 the 'clear' property only applies to block-level elements.
Therefore authors should only use this property on block-level
elements. If an implementation does support clear on inline elements,
rather than setting a clearance as explained above,
the implementation should force a break and effectively insert one or more empty line boxes (or shifting the new line box downward as described in section 9.5) to move the top of the cleared inline's line box to below the respective floating box(es).
In the absolute positioning model, a box is explicitly offset with
respect to its containing block. It is removed from the normal flow
entirely (it has no impact on later siblings). An absolutely
positioned box establishes a new containing block for normal flow
children and absolutely (but not fixed) positioned descendants. However, the contents of an
absolutely positioned element do not flow around any other boxes. They
may obscure the contents of another box (or be obscured themselves),
depending on the
stack levels of the overlapping boxes.
References in this specification to an absolutely positioned
element (or its box) imply that the element's 'position' property has the value
'absolute' or 'fixed'.
Fixed positioning is a subcategory of absolute positioning. The
only difference is that for a fixed positioned box, the containing
block is established by the viewport. For continuous media, fixed
boxes do not move when the document is scrolled. In this respect, they
are similar to fixed
background images. For paged media, boxes
with fixed positions are repeated on every page. This is useful for
placing, for instance, a signature at the bottom of each page.
Authors may use fixed positioning to create frame-like presentations.
Consider the following frame layout:
The three properties that affect box generation and layout —
'display',
'position', and
'float' — interact as follows:
If 'display'
has the value 'none', then
'position' and
'float' do not apply.
In this case, the element generates no box.
Otherwise, if 'position'
has the value 'absolute' or 'fixed', the box is absolutely positioned,
the computed
value of 'float' is 'none',
and display is set according to the table below.
The position of the box will be determined by the 'top', 'right', 'bottom' and 'left' properties and the box's
containing block.
Otherwise, if 'float' has a value other than 'none', the box is
floated and 'display' is set according to the table below.
Otherwise, if the element is the root element,
'display' is set according to the table below.
Otherwise, the remaining 'display' property values apply
as specified.
To illustrate the differences between normal flow, relative
positioning, floats, and absolute positioning, we provide a series of
examples based on the following HTML:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Comparison of positioning schemes</TITLE>
</HEAD>
<BODY>
<P>Beginning of body contents.
<SPAN id="outer"> Start of outer contents.
<SPAN id="inner"> Inner contents.</SPAN>
End of outer contents.</SPAN>
End of body contents.
</P>
</BODY>
</HTML>
The final positions of boxes generated by the outer and
inner elements vary in each example. In each illustration,
the numbers to the left of the illustration indicate the normal flow position of the double-spaced (for
clarity) lines.
Note. The diagrams in this section are illustrative and not to
scale. They are meant to highlight the differences between the
various positioning schemes in CSS 2.1, and are not intended to be
reference renderings of the examples given.
Consider the following CSS declarations for outer and
inner that don't alter the normal
flow of boxes:
#outer { color: red }
#inner { color: blue }
The P element contains all inline content: anonymous inline text and two SPAN
elements. Therefore, all of the content will be laid out
in an inline formatting context, within a containing block
established by the P element, producing something like:
#outer { position: relative; top: -12px; color: red }
#inner { position: relative; top: 12px; color: blue }
Text flows normally up to the outer element. The
outer text is then flowed into its normal flow position and
dimensions at the end of line 1. Then, the inline boxes containing the
text (distributed over three lines) are shifted as a unit by '-12px'
(upwards).
The contents of inner, as a child of outer, would
normally flow immediately after the words "of outer contents" (on line
1.5). However, the inner contents are themselves offset
relative to the outer contents by '12px' (downwards), back to
their original position on line 2.
Note that the content following outer is not affected by the
relative positioning of outer.
Now consider the effect of floating the
inner element's text to the right by means of the following
rules:
#outer { color: red }
#inner { float: right; width: 130px; color: blue }
Text flows normally up to the inner box, which is pulled
out of the flow and floated to the right margin (its 'width' has been assigned explicitly).
Line boxes to the left of the float are shortened, and the
document's remaining text flows into them.
To show the effect of the 'clear' property, we add a sibling
element to the example:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Comparison of positioning schemes II</TITLE>
</HEAD>
<BODY>
<P>Beginning of body contents.
<SPAN id=outer> Start of outer contents.
<SPAN id=inner> Inner contents.</SPAN>
<SPAN id=sibling> Sibling contents.</SPAN>
End of outer contents.</SPAN>
End of body contents.
</P>
</BODY>
</HTML>
The following rules:
#inner { float: right; width: 130px; color: blue }
#sibling { color: red }
cause the inner box to float to the right as before and the
document's remaining text to flow into the vacated space:
However, if the 'clear'
property on the sibling element is set to 'right' (i.e., the
generated sibling box will not accept a position next to
floating boxes to its right), the sibling content begins to
flow below the float:
#inner { float: right; width: 130px; color: blue }
#sibling { clear: right; color: red }
which cause the top of the outer box to be positioned with
respect to its containing block. The containing block for a positioned
box is established by the nearest positioned ancestor (or, if none
exists, the initial containing
block, as in our example). The top side of the outer box
is '200px' below the top of the containing block and the left side is
'200px' from the left side. The child box of outer is flowed
normally with respect to its parent.
The following example shows an absolutely positioned box that is a
child of a relatively positioned box. Although the parent
outer box is not actually offset, setting its 'position' property to 'relative'
means that its box may serve as the containing block for positioned
descendants. Since the outer box is an inline box that is
split across several lines, the first inline box's top and left edges
(depicted by thick dashed lines in the illustration below)
serve as references for 'top' and
'left' offsets.
the containing block for inner becomes the initial containing block (in our
example). The following illustration shows where the inner
box would end up in this case.
Relative and absolute positioning may be used to implement change
bars, as shown in the following example. The following fragment:
<P style="position: relative; margin-right: 10px; left: 10px;">
I used two red hyphens to serve as a change bar. They
will "float" to the left of the line containing THIS
<SPAN style="position: absolute; top: auto; left: -1em; color: red;">--</SPAN>
word.</P>
First, the paragraph (whose containing block sides are shown in the
illustration) is flowed normally. Then it is offset '10px' from the
left edge of the containing block (thus, a right margin of '10px' has
been reserved in anticipation of the offset). The two hyphens acting
as change bars are taken out of the flow and positioned at the current
line (due to 'top: auto'), '-1em' from the left edge of its containing
block (established by the P in its final position). The result is
that the change bars seem to "float" to the left of the current
line.
This integer is the stack level of the generated box
in the current stacking context. The box
also establishes a local stacking context in which its stack
level is '0'.
auto
The stack level of the generated box in the current stacking
context is the same as its parent's box. The
box does not establish a new local stacking context.
In this section, the expression "in front of"
means closer to the user as the user faces the screen.
In CSS 2.1, each box has a position in three dimensions. In addition
to their horizontal and vertical positions, boxes lie along a "z-axis"
and are formatted one on top of the other. Z-axis positions are
particularly relevant when boxes overlap visually. This section
discusses how boxes may be positioned along the z-axis.
The order in which the rendering tree is painted onto the canvas is
described in terms of stacking contexts. Stacking contexts can
contain further stacking contexts. A stacking context is atomic from
the point of view of its parent stacking context; boxes in other
stacking contexts may not come between any of its boxes.
Each box belongs to one stacking context. Each box in a given
stacking context has an integer stack level, which
is its position on the z-axis relative to other boxes in the same
stacking context. Boxes with greater stack levels are always formatted
in front of boxes with lower stack levels. Boxes may have negative
stack levels. Boxes with the same stack level in a stacking context
are stacked bottom-to-top according to document tree order.
The root element forms the root stacking context. Other stacking
contexts are generated by any positioned element (including
relatively positioned elements) having a computed value of 'z-index'
other than 'auto'. Stacking contexts are not necessarily related to
containing blocks. In future levels of CSS, other properties may
introduce stacking contexts, for example 'opacity'.
Each stacking context consists of the following
stacking levels (from back to front):
the background and borders of the element forming the stacking context.
the stacking contexts of descendants with negative stack
levels.
a stacking level containing in-flow non-inline-level
descendants.
a stacking level for floats and their contents.
a stacking level for in-flow inline-level descendants.
a stacking level for positioned descendants with 'z-index:
auto', and any descendant stacking contexts with 'z-index: 0'.
the stacking contexts of descendants with positive stack levels.
For a more thorough explanation of the stacking order, please see Appendix E.
The contents of inline blocks and inline tables are stacked as if
they generated new stacking contexts, except that any elements that
actually create new stacking contexts take part in the parent
stacking context. They are then painted atomically in the inline
stacking level.
In the following example, the stack levels of
the boxes (named with their "id" attributes) are:
"text2"=0, "image"=1, "text3"=2, and "text1"=3. The
"text2" stack level is inherited from the root box. The
others are specified with the 'z-index' property.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Z-order positioning</TITLE>
<STYLE type="text/css">
.pile {
position: absolute;
left: 2in;
top: 2in;
width: 3in;
height: 3in;
}
</STYLE>
</HEAD>
<BODY>
<P>
<IMG id="image" class="pile"
src="butterfly.png" alt="A butterfly image"
style="z-index: 1">
<DIV id="text1" class="pile"
style="z-index: 3">
This text will overlay the butterfly image.
</DIV>
<DIV id="text2">
This text will be beneath everything.
</DIV>
<DIV id="text3" class="pile"
style="z-index: 2">
This text will underlay text1, but overlay the butterfly image
</DIV>
</BODY>
</HTML>
This example demonstrates the notion of
transparency. The default behavior of the background is to allow boxes behind it to be visible.
In the example, each box transparently overlays the boxes below it. This
behavior can be overridden by using one of the existing
background properties.
Conforming user agents that do
not support bidirectional text may ignore the 'direction' and 'unicode-bidi' properties
described in this section.
The characters in certain scripts are written from right to
left. In some documents, in particular those written with the Arabic
or Hebrew script, and in some mixed-language contexts, text in a
single (visually displayed) block may appear with mixed
directionality. This phenomenon is called bidirectionality, or
"bidi" for short.
The Unicode standard ([UNICODE], section 3.11) defines a complex
algorithm for determining the proper directionality of text. The
algorithm consists of an implicit part based on character properties,
as well as explicit controls for embeddings and overrides. CSS 2.1 relies
on this algorithm to achieve proper bidirectional rendering. The 'direction' and 'unicode-bidi' properties allow
authors to specify how the elements and attributes of a document
language map to this algorithm.
If the rendered content contains right-to-left characters, and if the user
agent displays these characters in right-to-left order, the user
agent must apply the bidirectional algorithm.
(UAs that render right-to-left characters simply because a font on
the system contains them but do not support the concept of
right-to-left text direction are exempt from this requirement.)
Because the directionality of a text depends on the structure and
semantics of the document language, these properties should in most
cases be used only by designers of document type descriptions (DTDs),
or authors of special documents. If a default style sheet specifies
these properties, authors and users should not specify rules to
override them.
The HTML 4.0 specification ([HTML40], section 8.2) defines
bidirectionality behavior for HTML elements. The style sheet
rules that would achieve the bidi behavior specified in [HTML40] are
given in the sample style sheet. The
HTML 4.0 specification also contains more information on
bidirectionality issues.
This property specifies the base writing direction of blocks and
the direction of embeddings and overrides (see 'unicode-bidi') for the Unicode
bidirectional algorithm. In addition, it specifies the direction of table column layout, the direction of
horizontal overflow, and the
position of an incomplete last line in a block in case of 'text-align:
justify'.
Values for this property have the following meanings:
ltr
Left-to-right direction.
rtl
Right-to-left direction.
For the 'direction'
property to affect reordering in inline-level elements, the 'unicode-bidi' property's value
must be 'embed' or 'override'.
Note.
The 'direction' property, when
specified for table column elements, is not inherited by cells in the
column since columns are not the ancestors of the cells in the document tree.
Thus, CSS cannot easily capture the "dir" attribute inheritance rules described
in [HTML40], section 11.3.2.1.
Values for this property have the following meanings:
normal
The element does not open an additional level of embedding with
respect to the bidirectional algorithm. For inline-level elements,
implicit reordering works across element boundaries.
embed
If the element is inline-level, this value
opens an additional level of embedding with respect to the
bidirectional algorithm. The direction of this embedding level is
given by the 'direction'
property. Inside the element, reordering is done implicitly. This
corresponds to adding a LRE (U+202A; for 'direction: ltr') or RLE
(U+202B; for 'direction: rtl') at the start of the element and a PDF
(U+202C) at the end of the element.
bidi-override
For inline-level elements this creates an override.
For block-level, table-cell, table-caption, or inline-block elements
this creates an override for inline-level descendents not within
another block-level, table-cell, table-caption, or inline-block
element.
This means that inside the element, reordering is strictly in sequence
according to the 'direction'
property; the implicit part of the bidirectional algorithm is
ignored. This corresponds to adding a LRO (U+202D; for 'direction:
ltr') or RLO (U+202E; for 'direction: rtl') at the start of the
element and a PDF (U+202C) at the end of the element.
The final order of characters in each block-level element is the
same as if the bidi control codes had been added as described above,
markup had been stripped, and the resulting character sequence had
been passed to an implementation of the Unicode bidirectional
algorithm for plain text that produced the same line-breaks as the
styled text. In this process, non-textual entities such as images are
treated as neutral characters, unless their 'unicode-bidi' property has a
value other than 'normal', in which case they are treated as strong
characters in the 'direction'
specified for the element.
Please note that in order to be able to flow inline boxes in a
uniform direction (either entirely left-to-right or entirely
right-to-left), more inline boxes (including anonymous inline boxes)
may have to be created, and some inline boxes may have to be split up
and reordered before flowing.
Because the Unicode algorithm has a limit of
61 levels of
embedding, care should be taken not to use 'unicode-bidi' with a value other
than 'normal' unless appropriate. In particular, a value of 'inherit'
should be used with extreme caution. However, for elements that are,
in general, intended to be displayed as blocks, a setting of
'unicode-bidi: embed' is preferred to keep the element together in
case display is changed to inline (see example below).
The following example shows an XML document with bidirectional
text. It illustrates an important design principle: DTD designers should take bidi
into account both in the language proper (elements and attributes) and
in any accompanying style sheets. The style sheets should be designed
so that bidi rules are separate from other style rules. The bidi rules
should not be overridden by other style sheets so that the document
language's or DTD's bidi behavior is preserved.
Example(s):
In this example,
lowercase letters stand for inherently left-to-right characters and
uppercase letters represent inherently right-to-left characters:
Since this is XML, the style sheet is responsible for setting the
writing direction. This is the style sheet:
/* Rules for bidi */
HEBREW, HE-QUO {direction: rtl; unicode-bidi: embed}
ENGLISH {direction: ltr; unicode-bidi: embed}
/* Rules for presentation */
HEBREW, ENGLISH, PAR {display: block}
EMPH {font-weight: bold}
The HEBREW element is a block with a right-to-left base direction,
the ENGLISH element is a block with a left-to-right base
direction. The PARs are blocks that inherit the base direction from
their parents. Thus, the first two PARs are read starting at the top
right, the final three are read starting at the top left. Please note
that HEBREW and ENGLISH are chosen as element names for explicitness
only; in general, element names should convey structure without
reference to language.
The EMPH element is inline-level, and since its value for 'unicode-bidi' is 'normal' (the
initial value), it has no effect on the ordering of the text. The
HE-QUO element, on the other hand, creates an embedding.
The formatting of this text might look like this if the line length
is long:
Because HEBREW18 must be read before english19, it is on the line
above english19. Just breaking the long line from the earlier
formatting would not have worked. Note also that the first syllable
from english19 might have fit on the previous line, but hyphenation of
left-to-right words in a right-to-left context, and vice versa, is
usually suppressed to avoid having to display a hyphen in the middle
of a line.
