Attach styles to nodes in HTML import

[johno]

HTML elements can have attached style strings that should be converted to the internal style object. This should be relatively straightforward, though we need to write a CSS => schema converter which also handles parsing/tokenizing the values in the style/CSS declarations.

There can also be style tags which have CSS. We'll need to attach those to the relevant nodes. This is quite a bit trickier since we'll need to effectively handle selectors and style resolution. One idea I have regarding this is to render the HTML somewhere in the background (after sanitizing) and then using getComputedStyle to attach the relevant styles.