What version of HTML should I use?

The latest version of HTML is XHTML 1.0. XHTML stands for eXtensible Hypertext Markup Language and is a stricter and cleaner version of HTML
defined as an XML application.

XHTML is also more easily transferred between devices and as a result, is more accessible than HTML; users using screen readers or voice browsers can access your content more easily if it is written in XHTML.

There are currently 3 XHTML document types:

STRICT
TRANSITIONAL
FRAMESET

Web Pages should start with a Document Type Declaration saying that you want to use the transitional version of XHTML 1.0

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

We recommend that new web pages should use the TRANSITIONAL document type as it is intended to bridge the gap between HTML and XHTML and will work on old versions of web browsers that may still be in use within the university and beyond. We also recommend that you convert existing web pages to xhtml 1.0 transitional.

Differences between HTML and XHTML

XHTML is very similar to HTML 4.01. For more details, see the section headed Differences with HTML 4 at the w3c XHTML 1.0 pages.

Creating checking and converting to XHTML

You can create XHTML using Dreamweaver MX and you can also use this tool to convert existing HTML pages to XHTML transitional pages. Dreamweaver also has features for cleaning up and validating your XHTML markup.

You can also use the freely available HTML tidy utility to convert to XHTML 1.0 transitional.

To check that your XHTML is correct, use the w3c markup validation service.