layout: Support any `display` property in generated content, and allow
tables to clear floats.

Improves the GitHub header.
style: Tweak the UA style a little bit.
Merge pull request #3492 from pcwalton/clear-generated-content-table

layout: Support any `display` property in generated content, and allow

Reviewed-by: glennw
Fix some mutable variable warnings.
Merge pull request #3500 from glennw/warning-fix

Fix some mutable variable warnings.

Reviewed-by: pcwalton
Merge pull request #3499 from pcwalton/ua-fixes

style: Tweak the UA style a little bit.

Reviewed-by: SimonSapin
layout: Don't double-count padding, etc, and don't draw
backgrounds/borders/etc. twice for inline-blocks.

Improves Reddit, GitHub, and many other sites.
layout: Refactor a few things, mostly just to obey the line limit.