Currently, Validator.nu mines the HTML 5 spec for UI text describing permissible content models, element contexts and element-specific attributes. The text is shown when an element or attribute is misplaced on missing.
Unfortunately, the spec does not contain similarly extractable text for microsyntax descriptions. Microsyntaxes are syntaxes that appear mostly in attribute values—for example, HTML5 integer, Web Forms 2.0
week, RFC 2616 media types (aka. MIME types) or CSS3 Media Queries.
Based on IRC discussions, there is interest in producing the descriptions collaboratively. To that end, I have seeded the WHATWG wiki with a page for microsyntax descriptions. If you would like to help make validator messages better, please feel free to edit the wiki (under the MIT license).