One of the most important features of style sheets is that they
specify how a document is to be presented on different media: on the
screen, on paper, with a speech synthesizer, with a braille device,
etc.
Certain CSS properties are only designed for certain media (e.g.,
the 'page-break-before' property
only applies to paged media). On occasion, however, style sheets for
different media types may share a property, but require different
values for that property. For example, the 'font-size' property is useful both
for screen and print media. The two media types are different enough
to require different values for the common property; a document will
typically need a larger font on a computer screen than on paper.
Therefore, it is necessary to express that a style sheet, or a
section of a style sheet, applies to certain media types.
There are currently two ways to specify media dependencies for
style sheets:
Specify the target medium from a style sheet with the @media or @import at-rules.
Example(s):
@import url("fancyfonts.css") screen;
@media print {
/* style sheet for print goes here */
}
Specify the target medium within the document language. For
example, in HTML 4.0 ([HTML40]), the "media" attribute on the LINK
element specifies the target media of an external style sheet:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Link to a target medium</TITLE>
<LINK REL="stylesheet" TYPE="text/css"
MEDIA="print, handheld" HREF="foo.css">
</HEAD>
<BODY>
<P>The body...
</BODY>
</HTML>
An @media rule
specifies the target media types (separated
by commas) of a set of rules (delimited by curly braces). The @media construct allows style
sheet rules for various media in the same style sheet:
@media print {
body { font-size: 10pt }
}
@media screen {
body { font-size: 13px }
}
@media screen, print {
body { line-height: 1.2 }
}
The names chosen for CSS media types reflect target devices for
which the relevant properties make sense. In the following list of CSS
media types the names of media types are normative, but the
descriptions are informative. Likewise, the "Media" field in the
description of each property is informative.
all
Suitable for all devices.
braille
Intended for braille tactile feedback devices.
embossed
Intended for paged braille printers.
handheld
Intended for handheld devices (typically small
screen, limited bandwidth).
print
Intended for paged material and for documents viewed on
screen in print preview mode. Please consult the section on paged media for information about formatting
issues that are specific to paged media.
projection
Intended for projected presentations, for example projectors.
Please consult the section on paged media for
information about formatting issues that are specific to paged media.
screen
Intended primarily for color computer screens.
speech
Intended for speech synthesizers. Note: CSS2 had a similar media type
called 'aural' for this purpose. See the appendix on
aural style sheets for details.
tty
Intended for media using a fixed-pitch character grid (such as
teletypes, terminals, or portable devices with limited display
capabilities). Authors should not use pixel units with the "tty" media
type.
Media types are mutually exclusive in the sense that a user agent can
only support one media type when rendering a document. However, user
agents may use different media types on different canvases. For
example, a document may (simultaneously) be shown in 'screen' mode on
one canvas and 'print' mode on another canvas.
Note that a multimodal media type is still only one media
type. The 'tv' media type, for example, is a multimodal media
type that renders both visually and aurally to a single canvas.
Unknown media type names must not result in the @media rule being ignored.
Note.
Future versions of CSS may extend the list of media types. Authors
should not rely on media type names that are not yet defined
by a CSS specification.
Each CSS property definition specifies which media types the
property applies to. Since properties generally apply to several media
types, the "Applies to media" section of each property definition
lists media groups
rather than individual media types. Each property applies to all media
types in the media groups listed in its definition.
