The current consensus is that XHTML will replace HTML in the future. Learn more about XHTML by referring to the specification @ http://www.w3.org/TR/xhtml11
Some companies and organizations mandate the use of XHTML, namely, with a view of future standards.
You could use SXLT (XSL Transformations), to transform an XHTML document into another form. These features will not be available in HTML
Example that defines a web page that support XHTML 1.1.
<%@Page Language=”C#” AutoEventWireup=”true” CodeFile=”TestPage.aspx.cs” Inherites=”TestPage_aspx”%>
XHTML requires the definision of XML namespace for the element, which ASP.NET does not supply.
Visual Studio 2005 sets the XML namespace for the element and adds doctype for XHTML 1.1. You can change or remove doctype.
Doctype: XHTML 1.0 transitional
This doctype enforces the structural rules of XHTML but allows HTML formatting features that have been replaced by styleshteets and are considered obsolete.
The XHTML transitional doctype considers HTML frames obsolete. If you need to create a frame page, consider the XHTML 1.0 frameset doctype.
ASP.NET server controls will work equally well with and doctype and browser. You as the creator can specify the level of standard compliance.