HTML Levels

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.

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