That was exactly the first thing that came into my head too. The trouble with declaring a particular revision of HTML as 'supported' is that a standard as comp

That was exactly the first thing that came into my head too. The trouble with declaring a particular revision of HTML as 'supported' is that a standard as complex as HTML (or ECMAScript, or CSS) has many parts, and it's quite possible to correctly and completely support some parts and not others. For example, HTML 4 defines that tables can have a <col> element with an "align" attribute that declares the visual alignment of cells in that column. HTML 4 became a recommendation in December 1999, and yet I know of no browser that supports the "align" attribute on <col> elements. Does that mean that every browser has to mark HTML 4 as 'not supported'?