[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.

[svgeesus]

I will try to add the WPT annotations

[svgeesus]

I will try to add the WPT annotations

Done

[svgeesus]

Not quite fully implemented but getting there

Chrome 127: 5809/6195
Firefox 129: 5846/6206
Safari 17: 5778/6206
Edge 127: 5818/6206

[svgeesus]

Border image was part of Interop 2023

[svgeesus]

Background size is looking a bit rough, especially with vector images.