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
HTMLTidy is one of those wonderfully efficient little tools in the *nix tradition. Like the other tools of this type, it does only one thing, but does it well. It differs from most of the others in one respect though - it's name clearly describes what it does: tidy up messy html.
Have you ever worked on a project that involved editing html pages from a variety of sources? Ever had to work with an MSWord document saved as html? What about the chunderous mess that's spewed from some of those WYSIWYG web tools? Or maybe you create your pages the
HTML is based upon rules to create it and browsers use those rules to display the Web pages consistently. However, because HTML is written by people, there is a lot of possibility for error.
Programmers know that when they write C code they have programs like debuggers to find problems, and they also have a program called "lint" to check for errors in the code. The same is true for HTML writers.
Once you have written a Web page, you may notice that things you don't expect are happening. Unless you've written the page in a WYSIWYG editor, invalid