Version of the XHTML standard_CSS/HTML

There are three versions of XHTML 1.0, which correspond to three versions of HTML 4.01:

• XHTML 1.0 Transitional

• XHTML 1.0 Strict

• XHTML 1.0 Frameset

XHTML 1.0 Transitional includes all tags and attributes in HTML 4.01 Transitional. The XHTML 1.0 Transitional standard was introduced to enable existing HTML designers and developers to migrate to XHTML without much pain.

XHTML 1.0 Strict differs from XHTML 1.0 Transitional in that it enforces a clearer separation between document structure and presentation. Unlike XHTML 1.0 Transitional, XHTML 1.0 Strict forces you to use cascading style sheets to control the appearance of the page.

XHTML 1.0 Frameset documents are intended to be documents that use the tag to divide the browser into frames (XHTML 1.0 Transitional and Strict pages cannot contain the tag).

The W3C also published XHTML 1.1 as a Recommendation (May 31, 2001). XHTML 1.1 is very similar to XHTML 1.0 Strict. The main difference is that XHTML 1.1 can be extended with add-on modules to support new elements. For example, you can generate a specific XHTML 1.1 page that also contains elements from MathML (Mathematical Markup Language), SVG (Scalable Vector Language), or a custom module you create.

Finally, the W3C is developing an XHTML 2.0 recommendation. Because XHTML 2.0 is still in the drafting stage and no web browsers currently support the standard, we do not discuss it in this article.

ASP.NET 2.0 Framework and Visual Studio .NET 2005 target XHTML 1.0 Transitional. This standard is the least restrictive of the XHTML standards, and it is the most compatible with existing HTML pages. However, you can also generate ASP.NET 2.0 pages that target the XHTML 1.0 Strict standard or even the XHTML 1.1 standard (see the "Configuring XHTML conformance" section later).

(Note that Beta 2 versions of the ASP.NET Framework target XHTML 1.1 by default. The final version of the ASP.NET 2.0 Framework will target XHTML 1.0 Transitional.)