2.1. HTML

Three current standards:
1. HTML 4.01 Transitional/Strict.
2. XHTML 1.0 Transitional/Strict - corresponds to HTML 4.01, but based on XML.
3. XHTML 1.1 - more or less XHTML 1.0 Strict, with a few things done differently.
HTML 4.01 is better supported by older browsers, but XHTML can help detect a lot of bugs.
The Transitional variants may be recommended, because the strict ones disallows many features that are better supported by older or buggy browsers.