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Written by Diana, on 03-09-2007 21:18

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Media types

7 Media types

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7.1 Introduction to media types

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 'cue-before' property for aural user agents). 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. However, the two media 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. Experience also shows that sans-serif fonts are easier to read on screen, while fonts with serifs are easier to read on paper. For these reasons, it is necessary to express that a style sheet -- or a section of a style sheet -- applies to certain media types.

7.2 Specifying media-dependent style sheets

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.

@import url("loudvoice.css") aural;
@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>

The @import rule is defined in the chapter on the cascade.

7.2.1 The @media rule

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: 12pt }
  }
  @media screen, print {
    BODY { line-height: 1.2 }
  }

7.3 Recognized media types

A CSS media type names a set of CSS properties. A user agent that claims to support a media type by name must implement all of the properties that apply to that media type.

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 parenthetical descriptions are not normative. They only give a sense of what device the media type is meant to refer to.

all: Suitable for all devices.
aural: Intended for speech synthesizers. See the section on aural style sheets for details.
braille: Intended for braille tactile feedback devices.
embossed: Intended for paged braille printers.
handheld: Intended for handheld devices (typically small screen, monochrome, limited bandwidth).
print: Intended for paged, opaque 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 or print to transparencies. 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.
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.
tv: Intended for television-type devices (low resolution, color, limited-scrollability screens, sound available).

Media type names are case-insensitive.

Due to rapidly changing technologies, CSS2 does not specify a definitive list of media types that may be values for @media.

Note. Future versions of CSS may extend this list. Authors should not rely on media type names that are not yet defined by a CSS specification.

7.3.1 Media groups

Each CSS property definition specifies the media types for which the property must be implemented by a conforming user agent. Since properties generally apply to several media, 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.

CSS2 defines the following media groups:

continuous or paged. "Both" means that the property in question applies to both media groups.
visual, aural, or tactile.
grid (for character grid devices), or bitmap. "Both" means that the property in question applies to both media groups.
interactive (for devices that allow user interaction), or static (for those that don't). "Both" means that the property in question applies to both media groups.
all (includes all media types)

The following table shows the relationships between media groups and media types:

Relationship between media groups and media types
Media Types	Media Groups
	continuous/paged	visual/aural/tactile	grid/bitmap	interactive/static
aural	continuous	aural	N/A	both
braille	continuous	tactile	grid	both
emboss	paged	tactile	grid	both
handheld	both	visual	both	both
print	paged	visual	bitmap	static
projection	paged	visual	bitmap	static
screen	continuous	visual	bitmap	both
tty	continuous	visual	grid	both
tv	both	visual, aural	bitmap	both

Last update: 03-09-2007 21:18

Published in : Tools, CSS 2

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