Related: #2260.

We've still been trying to define CSS in terms of some never-defined abstract document tree, all the while doing things like 3f70d30 informatively referencing DOM because it's unclear how the DOM matches to our abstract data structure.

At this point, I'm unaware of any CSS implementation that doesn't construct the DOM or something black-box equivalent to it. It feels like we're keeping this abstraction around for no good reason?