[css-backgrounds-3] Steps to Recommendation

[svgeesus]

(Also css-backgrounds-3 is basically a REC that's just missing an implementation report so definitely anything other than an error correction goes in css-backgrounds-4.)

Originally posted by @fantasai in #6739 (comment)

[svgeesus]

The implementation report linked from the current spec points to the old, unmaintained, manually-run Shepherd testsuite, not to WPT. 128 tests are listed.

There are no WPT annotations. There are 763 tests (5556 subtests) in WPT, although these cover all levels.

I feel that the modern, maintained, automatically-run WPT tests are better in general to determine whether to advance to Proposed Rec. I understand that adding WPT annotations to a Bikeshed spec is a bunch of work, but it would be an improvement.