Hi Dale,
I definitely should have better documentation about that. Version 8 should have a better help system.
Here's how I would define the different message types:
Errors - (red) The most important issues most likely to cause problems for visitors. Errors can also be issues that are flagrantly against the standards.
Warnings - (yellow) Issues that may or may not cause problems for visitors, perhaps depending on the visitor's browser, but that you should know about. These may include issues against the standard but that are less likely to be problematic.
Message - (blue) Generally useful information that will probably not cause problems for visitors. Unlike comments, messages are usually tied to a specific part of the document so that clicking on them in the editor takes you to a specific part of the document.
Comments - (green) Similar to messages, but not usually tied to a specific section of the document. Comments typically give information (like the type, byte size, and download times of the document), or advice (like using an ICRA RDF label in the document).
Good luck convincing your boss that HTML should be correct (though it seems pretty logical to me)!
