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Beginning XHTML

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What Are Media Types?

One of the most exciting things about writing XHTML is that whatever you write can be viewed anywhere in the world, regardless of the type of computer being used to view the content. This means that whatever content you write, you had better be prepared for it to be viewed on any number of computers, television set-top boxes, game consoles, handheld digital assistants, and mobile phones.

 

This would be quite a chore had HTML continued its evolutionary path of including presentational elements like <b> for bold and <center> for alignment. Fortunately, as we saw earlier in the book, the World Wide Web Consortium were aware of this and invented Cascading Style Sheets to separate presentation (how the web page looks) from content (the stuff in the web page). This means that you can now have different styles of presentations for different devices, without affecting the content of your web page.

 

Each of these devices is categorized as belonging to a certain media type. Media types include things like 'screen', for computer monitors, 'print' for printed paper, and 'aural' for speech synthesizers.

Known Media Types and Media Type Descriptors

So far, we have used two media types: screen and print. The Cascading Style Sheets Level 2 specification identifies a handful of other media types and qualifies this list as not being considered definitive. With the explosions of Internet appliances, there are certain to be an increasing number of new media types.

Media Type Descriptor

Description

all

The style is suitable for all devices.

aural

The style is intended for speech synthesizers. In CSS-2, there are a handful of properties related to aural style sheets.

braille

The style is intended for braille tactile feedback devices.

embossed

The style is intended for paginated braille printers.

handheld

The style is intended for handheld devices.

print

The style is intended for paginated media (like the printed page) or when representing the printed page on a screen (such as a print preview mode).

projection

The style is intended for projectors and printing to film slides and transparencies.

screen

The style is intended for color computer screens.

tty

The style is intended for devices using a fixed-pitch character grid, such as teletypes.

tv

The style is intended for television-type devices.


As shown in the table above, Cascading Style Sheets Level 2 has ten media types that represent target devices for XHTML content. The names of these media types are called 'media type descriptors'. The media type descriptors are used as values for several case-sensitive attributes described below and must be in lower case.

Style Sheet Properties Appropriate to a Media Type

Not every style sheet property is appropriate for every target device. The volume property, for example, refers to what's known as the median volume of a waveform. This property is clearly related to aural media types (i.e. sound) and not to visually-oriented media types.

 

In fact, it turns out that there are relationships between the different media types. These relationships are called media groups, and the World Wide Web Consortium identifies four of these media groups:

 

            Continuous or paginated media

            Visual, aural, or tactile media

            Grid or bitmap media

            Interactive or static media

 

Each media type is a member of one or more of these media groups. For example, the screen media type is a member of the continuous, visual, bitmap, interactive, and static media groups.

 

Continuous or paginated media refers to the surface on which content is rendered. If the surface can always extend to hold all of the content, the media is considered continuous. If the surface has a limited size and additional surfaces must be created to hold all of the content, the media is said to be paginated. Computer screens are continuous and printed pages are paginated. We will talk about paginated media later in this chapter.

 

Visual, aural, or tactile media refers to how the information is presented to the user:

 

            If the user must use their eyes to consume the content, then the media is said to be visual.

            If the user must use their ears to consume the content, then the media is said to be aural.

            If the user must use touch to consume the content, then the media is said to be tactile.

 

Computer screens are visual, aural browsers are aural, and Braille printers are tactile.

 

Grid or bitmap media refers to how the visual or tactile information is rendered. If the information is rendered in a predefined grid layout (every 'character' has the same amount of space), then the media is said to be grid-based. If the information is rendered pixel by pixel (every 'character' uses whatever space it needs), then the media is said to be bitmap-based. Teletypes are grid-based and computer screens are bitmap-based.

 

Interactive or static media refers to whether the user can interact with the content. As would be expected, when the user can interact with the content, the media is said to be interactive. When the user cannot interact with the content, the media is said to be static. Computer screens are said to be interactive and printed pages are said to be static.

 

The table below summarizes the relationships between media types and media groups.

 

Media Types

Media Groups

 

Continuous or Paginated

Visual, Aural, or Tactile

Grid or Bitmap

Interactive or Static

aural

continuous

aural

neither

both

braille

continuous

tactile

grid

both

embossed

paginated

tactile

grid

both

handheld

both

visual

both

both

print

paginated

visual

bitmap

static

projection

paginated

visual

bitmap

static

screen

continuous

visual

bitmap

both

tty

continuous

visual

grid

both

tv

both

visual, aural

bitmap

both

 

Each Cascading Style Sheet property is only valid for one or more media groups. When you are tailoring a presentation to a particular media type, you need to identify the appropriate media group and determine whether you can control the desired property for that media group.

 


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©1999 Wrox Press Limited, US and UK.

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