The CSS linter was pretty hard to reason about. It was
pretty flexible about certain things, but then it would
prevent seemingly innocuous code from getting checked in.
This commit overhauls the pretty-printer to be more composable,
where every object in the AST knows how to render itself. It
also cleans up a little bit of the pre_fluff/post_fluff logic
in the parser itself, so comments are more likely to be "attached"
to the AST node that make sense.
The linter is actually a bit more finicky about newlines, but
this is mostly a good thing, as most of the variations before
this commit were pretty arbitrary.
Some email services will provide default styling for things like
buttons, and in this case the site created a style that had a
`background` attribute that overrode our `background-color` attribute
because of CSS importance heirarchy. This swiches us to use
`background` as well.
This also forces no `text-shadow` on buttons.
Fixes: #6775.
Abhinav Singh
Vishnu Ks
Cynthia Lin
Aditya Bansal
Steve Howell
Brock Whittaker