Walk the DOM and construct a parallel style tree that points back to the DOM
and has the relevant CSS property values hanging off of them.
The values are picked based on naive selector matching. There's no cascade
or specificity taken into account yet.
We walk the entire DOM and check all selectors against all elements. Only
id, class and tag name are checked right now. There's no ancestor stack
or compound selectors. All in good time :^)
