There are four official levels, or versions of HTML conformance. Each encompasses a set of tags and higher levels include tags from all those below it.
- Level 0
- The minimum tags which constitute an HTML document (most tags currently in use). Level 0 tags are usually rendered consistently from browser to browser.
- Level 1
- Level 0 tags plus tags for highlighting (also called Logical Tags) and images
- Level 2
- Level 0 and Level 1 tags plus form tags
- Level 3 (Version 3.2)
- Previous levels plus support for client-side image maps and scripts, and table markup elements.