XHTML defines elements and attributes more strictly than HTML. By default, all markup that is produced by ASP.NET and Web server controls included with ASP.NET now conforms to the XHTML 1.0 Transitional standard. In many cases, the markup produced by ASP.NET conforms to XHTML 1.1 standards as well. Unless noted otherwise, references to the XHTML standard in this topic mean both XHTML 1.0 and XHTML 1.1.

Some of the XHTML rules that differ from HTML include the following:

• All elements either include an explicit closing tag or are self-closing (with />).

• Tag and attribute names are rendered in lowercase, and attribute values are included in double quotation marks. For example, if you use a GridView control on your page, when the page is rendered, the GridView control emits HTML that is conformant with XHTML standards. All generated elements use explicit opening and closing tags (or self-closing tags), and attribute values are included in double quotation marks.

• Formatting information is rendered using only cascading style sheet styles. To support this standard, if the page includes an XHTML DOCTYPE element, ASP.NET controls do not render font elements or attributes, such as bgcolor, that would not conform to XHTML standards.

• In ASP.NET, if controls generate IDs, as occurs in the Repeater, GridView, and other controls, the format of the IDs match XHTML 1.0 Transitional guidelines.

• ASP.NET dynamically adds an action attribute to the form element. By default, form elements include a name attribute, which is allowed in the XHTML 1.0 Transitional specification. This helps maintain backward compatibility with existing applications which rely on client script that addresses form elements using the form name.

  Note:
  The name attribute on the form element is not allowed in XHTML 1.1 guidelines. You can configure your application not to render a name attribute. For details, see "Controlling XHTML Rendering of ASP.NET Pages and Controls" later in this topic.

• Because XHTML requires all elements to be enclosed in a container element, ASP.NET controls, such as input elements, are rendered in div elements. This includes the HTML markup rendered for controls, such as the TextBox, CheckBox, and RadioButton controls. It also includes hidden fields, such as the element that is used to store view-state data.

• ASP.NET encodes characters, such as & (for example, as &). This includes URLs that are generated to reference ECMAScript and the contents of encoded values, such as view state.

• Any script elements that are rendered into the page use the appropriate type attribute (for example, type="type/javascript") and do not include a language attribute. This pertains to scripts that are created by the page or controls that require client script to perform a postback — such as the HyperLink, LinkButton, Calendar, and TreeView controls, as well as validator controls — and by the RegisterHiddenField, RegisterStartupScript, and RegisterClientScriptBlock methods. Script blocks that you create are not automatically amended with a type attribute.

• If ASP.NET renders script blocks, the content of the script blocks is rendered inside an XML (HTML) comment.

Under some circumstances, you might want ASP.NET controls to render markup in the stricter format specified by the XHTML 1.1 specification. The default rendering includes some markup that does not conform to the XHTML 1.1 specification. For example, XHTML 1.1 standards prohibit the use of a name attribute in an HTML form element.

Conversely, you might want ASP.NET to render markup that does not conform to XHTML 1.0 Transitional specifications. This is typically true when you have existing pages that rely on tags or attributes that were supported in earlier versions of ASP.NET but do not conform to XHTML standards.

You can configure your Web site to render markup in three ways:

• Legacy (which is similar to how markup was rendered in previous versions of ASP.NET)

• Transitional (XHTML 1.0 Transitional)

• Strict (XHTML 1.0 Strict)

For more information, see How to: Configure XHTML Rendering in ASP.NET Web Sites.

Note:
The option to render legacy markup is provided primarily to assist you in migrating existing pages to the current version of ASP.NET, and might not be supported in future versions of ASP.NET.

Legacy Rendering

Valid XHTML Web pages must contain a DOCTYPE declaration that identifies the page as an XHTML page and references the XHTML schema to which it conforms. The page must also include attributes on the HTML tag that reference the XHTML namespace. ASP.NET does not automatically create a DOCTYPE declaration when the page is rendered. Instead, you should create the DOCTYPE declaration with the appropriate XML namespace references.

Note:
Visual designers, such as Visual Studio, generally include default page templates that include a DOCTYPE declaration. If you are using a visual designer, check that it creates new pages with the DOCTYPE declaration you require. For more information, see XHTML in Visual Web Developer.

The following code example shows a DOCTYPE declaration that you might add to your page.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html  >

By not automatically generating the DOCTYPE declaration, ASP.NET retains greater rendering flexibility for browsers that have differing levels of compliance with the differing HTML standards that can be specified in a DOCTYPE declaration.

Note:
If the DOCTYPE declaration is removed, XHTML compliance will not be met. The page will not be considered an XHTML page and will not reference the XHTML schema.

Many browsers will also change their rendering based on the presence or absence of the DOCTYPE declaration and an XML namespace declaration. If those elements are present, browsers typically use a standards-based rendering. When the elements are not present, many browsers render using browser-specific rules that vary among browser types (which is sometimes referred to as rendering in "quirks mode"), and can therefore result in unpredictable rendering.

Similarly, you can control the MIME type of the page. By default, a page sets the MIME type to text/html. However, you can override the page's MIME type by setting the ContentType attribute in the @ Page directive, as shown in the following code example.

<%@ Page Language="VB" ContentType="application/xhtml+xml" %>

<%@ Page Language="C#" ContentType="application/xhtml+xml" %>

Example ASP.NET Page with Required XHTML Elements

<%@ Page Language="VB" AutoEventWireup="false" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html >

<script runat="server">
Sub Button1_Click(sender As Object, e As EventArgs)
    Label1.Text = "Built at " & DateTime.Now.ToString()
End Sub

Sub listFruit_SelectedIndexChanged(sender As Object, e As EventArgs)
    Label1.Text = "You selected " & listFruit.SelectedItem.Text
End Sub
</script>

<head runat="server">
  <title>ASP.NET XHTML Page</title>
</head>

<body>
  <form id="Form1" runat="server">
    <div>
      <h1>ASP.NET Sample Page for XHTML</h1>
      <p>
      <asp:listbox runat="server" id="listFruit" AutoPostBack="true" 
          onselectedindexchanged="listFruit_SelectedIndexChanged">
         <asp:listitem>Apple</asp:listitem>
         <asp:listitem>Banana</asp:listitem>
         <asp:listitem>Orange</asp:listitem>
      </asp:listbox>
      </p>
      <asp:label runat="server" id="Label1" ForeColor="white" 
          BackColor="black" />
      <br />
      <asp:button runat="server" id="Button1" onclick="Button1_Click" 
          Text="Click me"/>
    </div>
  </form>
</body>
</html>

<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<script runat="server">
void Button1_Click(Object sender, EventArgs e)
{
    Label1.Text = "Built at " + DateTime.Now.ToString();
}
void listFruit_SelectedIndexChanged(Object sender, EventArgs e)
{
    Label1.Text = "You selected " + listFruit.SelectedItem.Text;
}
</script>

<html  >
<head runat="server">
  <title>ASP.NET XHTML Page</title>
</head>
<body>
  <form id="Form1" runat="server">
    <div>
      <h1>ASP.NET Sample Page for XHTML</h1>
      <p>
      <asp:listbox runat="server" id="listFruit" AutoPostBack="true" 
          onselectedindexchanged="listFruit_SelectedIndexChanged">
         <asp:listitem>Apple</asp:listitem>
         <asp:listitem>Banana</asp:listitem>
         <asp:listitem>Orange</asp:listitem>
      </asp:listbox>
      </p>
      <asp:label runat="server" id="Label1" ForeColor="white" 
          BackColor="black" />
      <br />
      <asp:button runat="server" id="Button1" onclick="Button1_Click" 
          Text="Click me"/>
    </div>
  </form>
</body>
</html>

ASP.NET does not alter static text or non-server HTML elements that you put into a page. For example, an ASP.NET Web page might include the TextBox and Button controls as well as some static text that you add inside <p></p> tags. ASP.NET can render XHTML for the TextBox and Button controls, but it cannot correct XHTML errors that occur between the <p></p> tags. If you create static text or HTML elements, make sure that they are XHTML conformant. You can check your pages by validating them, as explained in the next section.

Unknown attributes of HTML controls are passed through to the rendered control output and will not validate as valid XHTML markup. For example, specifying the ID attribute for the HtmlHead control will result in markup that is not XHTML 1.0 Strict-compliant. To validate your markup, use a markup validator such as the World Wide Web Consortium (W3C) Validation Markup Service.

After creating your ASP.NET Web pages, you might want to check whether they will render correct XHTML. If the page contains ASP.NET Web server controls, there is no way to check the page while writing it because the controls render XHTML only when the page runs.

Note:
Some visual designers, such as Visual Studio, can provide design-time XHTML validation of the page's markup.

To check the validity of the XHTML for your pages, you must use a service that runs the page and checks its output. A typical strategy is to deploy your pages to a publicly available server. The server can be a test server; it does not have to be a production server. However, it must be open to the Internet. You can then use a validation service that can read your pages programmatically.

A popular service is the W3C Markup Validation Service, which is maintained by the World Wide Web Consortium. To use this validator, enter the URL of the page that you want the service to check. The validation site requests the page and produces a report of any errors that it finds. Alternatively, you can save the source for a Web page and submit it as a file to the validation service. For more information about this validation service, see the W3C Web site.

If the page that you are checking contains dynamic content, or if users can personalize Web pages in your site, you must be sure to test pages with different content to be sure all possible content in the page is valid. In some cases, this can be difficult because the variation in possible page output is too great to be able to test effectively.

Configuring Browser Capabilities for Markup Validation

<browsers>
  <browser id="W3C_Validator" parentID="default">
    <identification>
        <userAgent match="^W3C_Validator" />
    </identification>
    <capabilities>
      <capability name="browser"              value="W3C Validator" />
      <capability name="ecmaScriptVersion"    value="1.2" />
      <capability name="javascript"           value="true" />
      <capability name="supportsCss"          value="true" />
      <capability name="tables"               value="true" />
      <capability name="tagWriter" 
         value="System.Web.UI.HtmlTextWriter" />
      <capability name="w3cdomversion"        value="1.0" />
    </capabilities>
  </browser>
</browsers>

Although ASP.NET generates XHTML–conformant markup, some controls support optional functionality that, if used, might result in non-compliant markup.

Note:
Each control renders its own markup. Custom controls created by other parties might not be written to produce XHTML-conformant output. If you are using a custom control, check with the control vendor to determine which standards the control supports.

Target Attribute

Select Element

Tasks