Techniques for WCAG 2.0

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SL31: Using Silverlight Font Properties to Control Text Presentation

Applicability

This technique relates to:

User Agent and Assistive Technology Support Notes

See User Agents Supported for general information on user agent support.

Description

The objective of this technique is to change the presentation / visual appearance of text, by setting several of the font-specific properties in the Silverlight API. Changing such properties does not change the semantic meaning of the text, nor does it alter the representation of the text that is available to assistive technologies through the Silverlight support of the UIA accessibility framework. By using font properties, it is possible to introduce a wide variety of presentation changes to fonts that do not introduce semantic elements that interfere with an assistive technology's view of text in the Silverlight application. In particular, adjusting font properties will make it possible to avoid any need for use of images of text, yet still provide a wide range of choices for text presentation.

Silverlight font properties exist on all controls, as well as on other text elements that are not true controls. For controls, the font properties apply in any case where the control enables a presentation mode that has enclosed text areas in its layout. By setting Silverlight font properties, it is possible to adjust presentation of font features without changing the structural connotation of that control, or the value of any control-specific property that contains plain-text. For example, the FontSize property can be set on a Paragraph (not a control) or on a Button (a control, and in this case the font size changes apply to any text displayed in the button content area). Font properties are also inheriting properties, meaning that if applying a font property value to a container in a relationship, those font property values can apply to child elements in the relationship. For example, if a FontSize is applied to a RichTextBox, that FontSize value is used by default by all the Paragraph items displayed in the RichTextBox.

Similar to CSS, Silverlight font properties can be grouped as a Style. That Style can be applied to all instances of a text element type (for example to all cases of Paragraph) or specifically referenced as a resource that is only used by certain instances of a text element type. Either way, the Style feature enables the separation of presentation from semantics for text elements, and enables workflows where content authors supply the semantic text and design-oriented authors adjust the related Silverlight styles. For more information on the Silverlight concept of styles, see Control Customization on MSDN.

The following Silverlight font properties are useful to style text and avoid the need for text in images. Links in this list refer to the Control class version of these properties.

So long as images of text are avoided, the text within a Silverlight text element can be reported to the UI Automation accessibility framework that Silverlight supports. That text is reported using the same basic text content as is used for semantic text display in the UI. In other words, exposing that text to assistive technologies that use UIA as a framework does not require the Silverlight application author to resort to automation-specific override properties such as AutomationProperties.HelpText; the automation peers for text elements report all necessary text content to automation as a built-in behavior of the text element controls. For more information on UI Automation and text containers, see SL32: Using Silverlight Text Elements for Appropriate Accessibility Role.

CSS versus Silverlight font properties

Related CSS techniques mention that users can override any page-declared CSS styling techniques, by invoking browser-specific features. For example, using Internet Explorer, a user can use Tools / Internet Options, Appearance / Accessibility to override certain classifications of CSS-controlled font properties when displaying HTML documents, or to use a user-specific style sheet for HTML documents. No browser-level equivalent feature exists for user alteration of Silverlight text properties in the Silverlight content area. Instead, application authors could supply controls that enable similar font-property changing behavior, and include those controls in the application-specific user interface. For more information on this technique, see SL13: Providing A Style Switcher To Switch To High Contrast.

Glyphs

Silverlight API includes a related text presentation API Glyphs. Glyphs is intended for specific decorative or niche language-support scenarios. The Glyphs API does not offer as much UIA exposure or the ability to programmatically change typical font properties; the main scenarios for Glyphs are to package migrated text content from document formats, or for purely decorative text in a font that is not commonly found on a user system and only the glyphs actually used in the Unicode string are subsetted into the Glyphs font payload. If addressing the WCAG criteria, authors should avoid using Glyphs API and instead use other text containers such as TextBox, along with a font that is supplied in the application package or known to exist on the end user system.

Examples

Example 1: Run time applied font properties, style, and template

This example illustrates applying runtime changes to a font property.

This example has UI in XAML, and logic in C#. The following is the XAML. 

<UserControl x:Class="DocumentStructure.MainPage"
   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>
   <UserControl.Resources>
       <Style x:Key="NewStyle" TargetType="Control">
           <Setter Property="FontFamily" Value="Arial"/>
           <Setter Property="FontSize" Value="30"/>
           <Setter Property="Height" Value="40"/>
       </Style>
   </UserControl.Resources>
   <StackPanel x:Name="LayoutRoot" Background="White">
   <RichTextBox IsReadOnly="True" Name="rtb" FontFamily="Algerian" FontSize="20">
           <Paragraph>Call me Ishmael. Some years ago--never mind how long precisely--having little or no money in my purse, 
           and nothing particular to interest me on shore, I thought I would sail about a little 
and see the watery part of the world. It is a way I have of driving off the spleen and 
regulating the circulation. Whenever I find myself growing grim about the mouth; whenever 
it is a damp, drizzly November in my soul; whenever I find myself involuntarily pausing before 
coffin warehouses, and bringing up the rear of every funeral I meet; and especially whenever my 
<Hyperlink NavigateUri="http://en.wiktionary.org/wiki/hypo">hypos</Hyperlink>
get such an upper hand of me, that it requires a strong moral principle to prevent me from deliberately stepping into 
the street, and methodically knocking people's hats off--then, I account it high time to get to sea as soon as I can. 
This is my substitute for pistol and ball. With a philosophical flourish Cato throws himself 
upon his sword; I quietly take to the ship. There is nothing surprising in this. If they but knew it, 
almost all men in their degree, some time or other, cherish very nearly the same 
feelings towards the ocean with me.
           </Paragraph>
           <Paragraph>There now is your
               <Hyperlink 
               NavigateUri="http://en.wikipedia.org/wiki/New_York_Harbor">insular city of the Manhattoes</Hyperlink>
, belted round by wharves as Indian isles by coral reefs--commerce surrounds it 
with her surf. Right and left, the streets take you waterward. Its extreme downtown is the 
battery, where that noble mole is washed by waves, and cooled by breezes, which a few hours 
previous were out of sight of land. Look at the crowds of water-gazers there.
           </Paragraph>
           <Paragraph>Circumambulate the city of a dreamy Sabbath afternoon. 
Go from Corlears Hook to Coenties Slip, and from thence, by Whitehall, northward. What do you see?
--Posted like silent sentinels all around the town, stand thousands upon thousands of mortal men 
fixed in ocean reveries. Some leaning against the spiles; some seated upon the pier-heads; 
some looking over the bulwarks of ships from China; some high aloft in the rigging, as if striving 
to get a still better seaward peep. But these are all landsmen; of week days pent up in lath 
and plaster--tied to counters, nailed to benches, clinched to desks. How 
then is this? Are the green fields gone? What do they here?
           </Paragraph>
       </RichTextBox>
       <Button Name="swapper" Click="swapper_Click" Width="220">Swap styles</Button>
   </StackPanel>
</UserControl>

The following is C# code: 

       private void swapper_Click(object sender, RoutedEventArgs e)
       {
           rtb.Style = this.Resources["NewStyle"] as Style;
       }

This example is shown in operation in the working example of Document Structure.

Resources

Resources are for information purposes only, no endorsement implied.

Tests

Procedure

  1. Using a browser that supports Silverlight, open an HTML page that references a Silverlight application through an object tag.

  2. Test that application of font properties as enabled in application UI changes presentation, but does not change semantic meaning of text.

  3. Close the browser. Repeat the test with an accessibility framework test tool running. There should be no difference in the structure or relationships in the accessibility view beyond the presentation changes.

Expected Results

#2 and #3 are true.

If this is a sufficient technique for a success criterion, failing this test procedure does not necessarily mean that the success criterion has not been satisfied in some other way, only that this technique has not been successfully implemented and can not be used to claim conformance.

Techniques are Informative

Techniques are informative—that means they are not required. The basis for determining conformance to WCAG 2.0 is the success criteria from the WCAG 2.0 standard—not the techniques. For important information about techniques, please see the Understanding Techniques for WCAG Success Criteria section of Understanding WCAG 2.0.

This Web page is part of Techniques and Failures for Web Content Accessibility Guidelines 2.0. The entire document is also available as a single HTML file. See the The WCAG 2.0 Documents for an explanation of how this document fits in with other Web Content Accessibility Guidelines (WCAG) 2.0 documents. To send public comments, please follow the Instructions for Commenting on WCAG 2.0 Documents.

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