[css-backgrounds-3] Computed value of background-repeat includes two values

[csnardi]

According to https://drafts.csswg.org/css-backgrounds-3/#background-repeat, the computed value of background-repeat is:

A list, each item consisting of: two keywords, one per dimension

However, I believe no browser follows this part of the spec (see https://wpt.fyi/css/css-backgrounds/background-332.html, where round is computed to round instead of round round). This was changed in a3493c4 in response to http://lists.w3.org/Archives/Public/www-style/2012Feb/1094.html from @dbaron, but I believe #1564 flipped that by suggesting the shorter serialization is preferred.

Related to #2574.

[dholbert]

As noted in #2574 (comment) and the subsequent CSSWG resolution, the CSSWG says that the specced "computed value" is not intending to describe the serialization. See also #4060

So, this issue here probably wants the same fate as #2574. CC @fantasai

[tabatkins]

Closing, as #4060's edits now make it clear that the "Computed Value" lines are not serialization instructions.