WPT details: * https://wpt.fyi/results/css/css-backgrounds/parsing?label=experimental&label=master&aligned * https://wpt.fyi/results/css/css-backgrounds/animations/background-position-interpolation.html?label=experimental&label=master&aligned * https://wpt.fyi/results/css/css-backgrounds/animations/background-size-interpolation.html?label=experimental&label=master&aligned * https://wpt.fyi/results/css/css-masking/parsing?label=experimental&label=master&aligned
Makes the computed value of the background-* longhands keep the number of comma-separated values the author specified, instead of repeating or truncating it to the background-image layer count. Repeating and truncating are used-value behavior: "The computed values of the coordinating list properties are not affected by such truncation or repetition." This feature implements the behavior defined in the https://drafts.csswg.org/css-values-4/ Appendix A.
The behavior defined in the "Appendix A: Coordinating List-Valued Properties" of the CSS Values And Units L4 specification has been discussed recently and agreed that it should stand. This makes Chrome the only browser diverging in the specified behavior.