Write complete spec-compliant rule for merging `style` attributes

[ef4]

TODO: we need a more complete definition of the style merging rule that cover the whole spec for what is legal within style attributes. See https://www.w3.org/TR/css-syntax-3/, in particular the parse a list of declarations parser entry point, which is the one relevant to HTML style attributes. Thankfully this only pulls in a subset of the entire CSS syntax.