Compatability tests for one browser / user agent against a markup language framework / source collection / input. It's leverages a qualitative analysis of "intent", created from the source and implied abilities from the user agent. This "intent" measure is somewhat opinionated, but it depends upon the default behavior of browsers and the validation of w3c code / Acid test matching. So it's influcenced by standards for the code, yet also influenced by the bugs in interpreters. This measures compliance between the two parties - the author and observer.