The Linux Quality Database is the
Free Hosting Service HTML Validation Test Page. This aims to combat the problem of free website hosting services inserting advertising markup that causes otherwise W3C-compliant web pages to become invalid.
Valid HTML is important not just to ensure interoperability with all web browsers such as those used on Linux, but to allow accessibility of websites to the blind.
What anyone can do is copy the page, a valid XHTML 1.0 strict document which is published under the GNU Free Documentation License, to a free web hosting service. You then click a link in it to see if the markup added by the free hosting service still allows the page to validate. You can compare this to validating the original document by clicking another link.
The page is more than just a markup test: it explains to the hosting services why it was placed on their server, why it is important for everyone to write valid markup, where to find several validators and how to get started writing valid HTML. Finally, it gives instructions anyone may use to copy the original document to a new hosting service for testing.
You can find an example of the test page at a free hosting service whose advertising markup makes it invalid by