This one represents one secton of a `background` property, since it can
have multiple background values separated by commas. Eventually, we will
represent that as a List of BackgroundStyleValues.
Also modified some background-foo properties in StyleResolver so that
the is_background_x() functions could be removed.
I realized that our handling of var() in shorthand properties is wrong,
so have been removing the is_builtin_or_dynamic() calls from the parsing
code for shorthands. This broke our var() test page, so I have replaced
the use of 'background' with 'background-color' there.
This adds test pages for border-radius, CSS custom properties and
flexboxes to the default page in the Browser.
I used those files to develop said features and they can be of use
when debugging in the future or just to showcase those features.
