I dispute that people care about 100% compatibility.
>So, they way to summarise compliance will be [...] some list of CSS features and Yes/No tick marks. //
I think you misunderstand the meaning of "summarise", "this engine is 90% CSS3 compatible" is a summary. Quirksmodes tables - http://www.quirksmode.org/css/contents.html - whilst useful during development aren't really something you can put before a user as an equivalent to an "HTML5" badge.
In CSS2 there are about 500 or so different things that you'd need to list (http://www.w3.org/TR/CSS2/indexlist.html).
I dispute that people care about 100% compatibility.
>So, they way to summarise compliance will be [...] some list of CSS features and Yes/No tick marks. //
I think you misunderstand the meaning of "summarise", "this engine is 90% CSS3 compatible" is a summary. Quirksmodes tables - http://www.quirksmode.org/css/contents.html - whilst useful during development aren't really something you can put before a user as an equivalent to an "HTML5" badge.