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Making an Web Site AccessibleScott MitchellMay 2004Summary: Take advantage of inheritance in the .NET Framework to extend classes to make them generate code that is fully accessible to people with disabilities. (13 printed pages) IntroductionWhile the majority of online users are able to browse Web sites using Web browsers with the typical browser settings, users with disabilities commonly use alternative means to access online information. For example, a blind user might use a text-only browser that converts the text into Braille, or reads the text aloud using a screen reader. A person with reduced vision might still use a browser like Microsoft&reg; Internet Explorer, but with a screen magnifier, or with the browser configured to use extra large font sizes. Motor disabilities might preclude a user from using the mouse or keyboard as an input device.Since people with disabilities typically surf the Web using special devices or non-standard browser configurations, a Web site's overall design and HTML markup greatly impacts the disabled person's user experience. For example, specifying font sizes using absolute measurements—like 10pt—displays the font in that absolute size, and not relative to the text size the user has configured in his browser. Sites that have a Flash or Shockwave interface, and do not provide an alternate, text representation, effectively cut off users with Braille devices or text-to-speech synthesizers. A Web site is said to be accessible if it is designed to transition gracefully to alternate devices.At this point, you might be thinking, "Why should I bother ensuring my Web site is accessible?" There are two good reasons I can think of:1.It's Good Business – According to the U.S. Census 2000, 49.7 million Americans have a disability; a June 2000 poll by Harris Interactive shows that 43% of disabled Americans are regular online users, and that users with disabilities spend almost twice as much time online than users without disabilities. Put the results of these two surveys together and you'll find that there are more than 21 million Americans with disabilities who are regularly online. By not taking the time to make your Web site accessible, you are cutting off 21 million potential visitors.2. It's a Mandate for Government Agencies – In 1998 the United State government passed Section 508 of the Rehabilitation Act, requiring Federal agencies to make their electronic information accessible to individuals with disabilities. This law provides accessibility guidelines for software applications andWeb applications, as well as telecommunication products and video products. Not only are Federal agencies required to implement accessibility guidelines, but also private companies that are contracted to work for the Federal government. (A number of countries outside the U.S. also have similar accessibility requirements for government agencies.) Therefore, creating accessible Web applications is required if you work for the government or for a company that provides services to the government.In this article we will discuss what steps you can take to ensure that your Microsoft® Web site is accessible. We'll take a brief look at the official accessibility guidelines available today, and then focus in on the accessibility guidelines used by the United States government. The article wraps up with a look at how to use inheritance to turn non-accessible Web controls into ones that meet accessibility guidelines.The WAI, WCAG, and Section 508There are a number of steps that can be taken to make a Web site more accessible. But what, exactly, are these steps, and how many of them does one site need to employ in order to be considered accessible? The answers to these questions differ based on who you ask, and what level of accessibility they need to provide. An intranet site for a Fortune 500 company would likely require a higher degree of accessibility than an intranet site for a company with only 25 employees.The WC3's Say on AccessibilityTo help formalize this discussion, in 1999 the WC3 officially founded the Web Accessibility Initiative (WAI), a group tasked with improving Web site usability for people with disabilities. The WAI's first act was to publish the Web Content Accessibility Guidelines, or WCAG. The WCAG provides a list of 14 guidelines for accessible Web site design. The guidelines themselves do not spell out actions to take to make a Web site more accessible. Rather, they are high-level statements that provide comment on how to ensure accessibility. For example, guideline 1 is "Provide equivalent alternatives to auditory and visual content." Accompanying each guideline is a set of checkpoints. The checkpoints spell out actions that can be taken to ensure that the accessibility guideline is met. Each checkpoint is given one of the following priority values:• Priority 1 – a Web developer must satisfy this checkpoint, otherwise one or more groups of users will not be able to access the content.• Priority 2 – a Web developer should satisfy this checkpoint, otherwise one or more groups of users will find it difficult to access the content.• Priority 3 –a Web developer may address this checkpoint, otherwise one or more groups of users might have difficulty when accessing the content.In addition to listing the 14 guidelines and their associated, prioritized checkpoints, the WCAG also provides a three-level classification scale for Web site accessibility. Sites that implement all Priority 1 checkpoints are rated at conformance level A. Those that implement all Priority 1 and Priority 2 checkpoints are rated Double-A, while those that implement all of the checkpoints are rated Triple-A.The WCAG gives a good set of actions to perform to ensure varying levels of accessibility.Although a thorough discussion of the WCAG is beyond the scope of this article, listed below are the 14 high-level guidelines of the WCAG. The checkpoints for each guideline, and their associated priority, can be found at the official Web Content Accessibility Guidelines 1.0 specification.15. Provide equivalent alternatives to auditory and visual content16. Don't rely on color alone17. Use markup and style sheets and do so properly18. Clarify natural language use19. Create tables that transform gracefully20. Ensure that pages featuring new technologies transform gracefully21. Ensure user control of time-sensitive content changes22. Ensure direct accessibility of embedded user interfaces23. Design for device-independence24. Use interim solutions25. Use W3C technologies and guidelines26. Provide context and orientation information27. Provide clear navigation mechanisms28. Ensure that documents are clear and simpleMicrosoft's Accessible Web Controls for 1.x.Many of the WCAG checkpoints and Section 508 rules dictate specific HTML markup that should be used to make a Web site more accessible. For example, checkpoint 5.1 in the WCAG (a Priority 1 checkpoint) and rule (g) in Section 508 requires that <table> elements properly identify row and column headers. That is, the <td> element must be used to identify data cells, whereas <th> must be used to identify headers.When building Web sites, developers rarely have to worry about generating HTML markup. Rather, Web controls are used that emit the proper markup. Unfortunately, many of the Web controls emit markup that violates the accessibility rules outlined by the WCAG and Section 508. Since all Web controls are, fundamentally, classes in the Microsoft&reg; .NET Framework, their functionality can be extended in a number of ways to conform to accessibility guidelines.A prime example of enhancing an existing Web control to meet accessibility guidelines can be seen with the DataGrid Web control. The DataGrid Web control that ships with the .NET Framework does not conform to rule (g) in Section 508 / checkpoint 5.1 in the WCAG. That is, the headers are rendered using <td> elements instead of <th>. (To see what I mean, check out this live demo, and do a View/Source in your browser. You'll see that the header row uses <td>s rather than <th>s.) However, in June 2003 Microsoft released an Hotfix Rollup Package that, among other things, includes an improved version of the DataGrid, one that conforms to rule (g) in Section 508.Note The Hotfix Rollup Package provides other accessibility-related enhancements. Forexample, it adds an optional AssociatedControlID property to the Label Web control to specify the ID of the Web control the Label is associated with. If this property is provided, the Label is rendered as a <label> HTML element with the for attribute referencing the specified Web control. For more information consult this knowledge base article.Creating An Adaptive, Accessible DataGrid Web ControlThe Web controls provided in the June 2003 Hotfix bring the Web controls up to Section 508 standards. Even with the Hotfix, however, there are still a number of WCAG Priority 1 checkpoints not being met. Fortunately you can easily extend the functionality of existing Web controls to conform to the accessibility guidelines required by your company. This is possible thanks to the power of inheritance. Specifically, we can take a Web control that does not conform to the accessibility standards, and create an extended Web control that does conform to the standards. By using inheritance, we only have to change or add the functionality to make the control conforming to the standards, and do not have to rewrite the base functionality.WCAG Checkpoint 6.3 reads: "Ensure that pages are usable when scripts, applets, or other programmatic objects are turned off or not supported. If this is not possible, provide equivalent information on an alternative accessible page. For example, ensure that links that trigger scripts work when scripts are turned off or not supported (for example, do not use "javascript:" as the link target)." However, the LinkButton Web control is rendered as a hyperlink with a javascript: link target (<a href="javascript:__doPostBack('...','...')">...</a>). The problem is that browsers that don't support JavaScript won't be able to postback the Web page by clicking on the LinkButton. This renders sortable DataGrids unsortable in browsers that don't support JavaScript since the column headers are rendered as LinkButtons.The remainder of this article looks at building a set of adaptive custom DataGrid columns that will display a Button Web control in the header of a sortable DataGrid if it is visited by a browser that doesn't support JavaScript. The Button Web control works for browsers that do not support JavaScript since it renders a <input type="submit"> tag rather than a hyperlink with a javascript: link target.Note Section 508 does not require that links not use the javascript: link target. Section 508's sole comment on client-side script can be found in rule (l), which reads: "pages utilize scripting languages to display content, or to create interface elements, the information provided by the script shall be identified with functional text that can be read by assistive technology." What this rule requires is that if content is generated by script, or if a page's behavior is modified via script, that a <noscript> HTML element be used with an explanation of the script behavior. Regardless, I have heard from several developers working on government projects that required them to support browsers without JavaScript functionality. Making DataGridColumn AdaptiveThe DataGrid Web control is comprised of a number of columns, which are classes that derive from the System.Web.UI.WebControls.DataGridColumn class. ships with five built-in DataGridcolumn types:1. BoundColumn2. ButtonColumn3. EditCommandColumn4. HyperLinkColumn5. TemplateColumnThe base class, DataGridColumn, provides the properties and methods inherent to all DataGrid column types. These include properties like HeaderText, SortExpression, Visible, and others. The DataGridColumn class has two methods that play an important role in creating a DataGrid: •Initialize() –initializes the DataGridColumn. Called once per column during the DataGrid's databinding process.•InitializeCell(cell, index, itemType) – called for each cell that is created during for each column during the DataGrid's databinding process. Cell is a TableCell object representing the actual HTML <table> cell for the cell being initializes; index is the row index of the cell being initialized; itemType is value from the ListItemType enumeration, specifying what type of cell is being added (Item, AlternatingItem, Header, Footer, EditItem, and so on.The DataGridColumn class is responsible for rendering the header and footer for the column. The derived classes –BoundColumn, ButtonColumn, EditCommandColumn, etc. –are responsible for rendering the DataGrid items.Note For more information on creating custom DataGrid columns, check out Marcie Robillard's article Creating Custom Columns for the DataGrid.To have our DataGrid adaptively render the header based on whether or not the user's browser supports JavaScript, we need to create a custom DataGrid class that is derived from DataGridColumn. This custom DataGrid column class, named AccessibleDataGridColumn, will need to override the DataGridColumn class's InitializeCell() method, checking to see if the DataGrid is configured for sorting and being visited by a browser that doesn't support JavaScript. If these two conditions are true, then AccessibleDataGridColumn will render a Button in the header as opposed to a LinkButton. The code for this class is shown below:Public Class AccessibleDataGridColumnInherits DataGridColumnPublic Overrides Sub InitializeCell( _ByVal cell As TableCell, _ByVal columnIndex As Integer, _ByVal itemType As ListItemType)Dim sorting As Boolean = _Not Me.Owner Is Nothing _AndAlso Me.Owner.AllowSorting _AndAlso Me.SortExpression.Length > 0Dim supportsJS As Boolean = _Not HttpContext.Current Is Nothing _AndAlso HttpContext.Current.Request.Browser.JavaScriptIf sorting And Not supportsJS _And itemType = ListItemType.Header And Not Me.DesignMode _Then If Me.HeaderImageUrl.Length > 0 ThenDim b As New ImageButtonb.AlternateText = Me.HeaderTextb.ImageUrl = Me.HeaderImageUrlmandName = "Sort"mandArgument = Me.SortExpressionb.CausesValidation = False cell.Controls.Add(b)ElseDim b As New Buttonb.Text = Me.HeaderTextmandName = "Sort"mandArgument = Me.SortExpressionb.CausesValidation = Falsecell.Controls.Add(b)End IfElse MyBase.InitializeCell(cell, columnIndex, itemType)End IfEnd SubEnd ClassThe overridden InitializeCell() method checks to see if sorting is enabled, if the current browser does not support client-side JavaScript, if the current item being initialized is a header, and if the rendering is not being performed in Microsoft&reg; Visual Studio&reg; .NET's designer. If this check passes, then another check is performed to determine if the column has a HeaderImageUrl value specified. If it does, it renders an ImageButton, setting its AlternateText to the column's HeaderText. If there is no HeaderImageUrl specified, a Button is created and added to the cell.Note Notice that the check to see whether or not a browser supports JavaScript simply looks up Request.Browser.JavaScript property. This property is set based on the User-Agent string sent by the browser to the Web server. The adaptive DataGrid will render Buttons in the header for a sortable DataGrid when someone visits with a browser that is known not to support JavaScript, such as Netscape 1.0, or Lynx. If someone visits with Microsoft&reg; Internet Explorer 6.0 it will render the defaultLinkButtons, even if the visitor has manually turned off JavaScript support.Using the Accessible DataGrid Column Classes in an Web PageThis article's download includes two Microsoft&reg; Visual Studio&reg; .NET 2003 projects (both Visual Basic .NET projects): the first is a class library that contains the custom DataGrid column classes; the second is an Web application that you can use to test out the adaptive, accessible DataGrid column classes.To use these custom DataGrid column classes in an Web project you need to first add the compiled assembly of the custom DataGrid column classes to the Web project's /bin folder. (If you are using Visual Studio .NET, the simplest way to do this is to add the assembly to the References folder in the Solution Explorer.) Next, you need to add a @References directive at the top of the Web pages that will utilize these DataGrid columns like so:<%@ Register TagPrefix="accessibility" Namespace="AccessibilityControls" Assembly="AccessibilityControls" %>Finally, to use the columns, add a DataGrid to the Web page and set the AutoGenerateColumns property to False. In the HTML portion, add the following syntax:<asp:DataGrid runat="server" AutoGenerateColumns="False" ...> <Columns> <accessibility:AccessibleBoundColumn DataField="FieldName" ...> </accessibility:AccessibleBoundColumn> ... </Columns></asp:DataGrid>That's all there is to it! Figure 3 shows a screenshot of the HTML portion of an Web page that uses the AccessibleBoundColumn and AccessibleButtonColumn DataGrid column classes. Figure 4 shows a screenshot of the design view of that same page.Figure 3. Using the AccessibleBoundColumnFigure 4. AccessibleBoundColumn in the designerNotice that in the Visual Studio .NET Designer the DataGrid with the custom, accessible columns appears just like normal. Too, if you view the page through a browser that supports JavaScript (as Figure 5 shows), the DataGrid will be rendered as it normally is, with LinkButtons in the column headers and ButtonColumn. However, if you visit the page with a browser that doesn't support JavaScript, such as Netscape 1.0, the ButtonColumn and column headers will be rendered as Buttons (see Figure 6).Figure 5. AccessibleBoundColumn in up-level browserFigure 6. AccessibleBoundColumn in older browserAccessibility in WhidbeyThe server controls that ship in the next version of , codenamed Whidbey, will automatically generate Section 508-compliant markup by default. Furthermore, these server controls will emit WCAG-compliant markup as well. To top it off, Microsoft&reg; Visual Studio&reg; .NET2005 will ship with an add-in to automatically check HTML markup for Section 508 and WCAG-compliance. For more information on these exciting additions, check out XHTML and Accessibility in Whidbey, a blog post from Scott Guthrie, co-founder of the team at Microsoft.For more information about Web site accessibility, be sure to check out Microsoft's Accessibility home page. There's also a Section 508 page as well, with resources on how various Microsoft products meet Section 508 standards.Related Books• Data Web Controls Kick Start• Unleashed•Web Accessibility for People with Disabilities•Accessibility for Everybody: Understanding the Section 508 Accessibility Requirements使一个 网站容易接近和理解①作者：Scott Mitchell②出版时间：May 2004摘要: 利用类的可继承性使得它们的代码可以被那些没有能力的人用和理解。