According to [1], setting color-scheme with the meta element sets a presentational hint on the root element which should cause the computed value of the root element to be affected. The computed value for the root element is always 'auto' in WebKit. This causes a bunch of wpt failures in https://wpt.fyi/results/css/css-color-adjust/meta [1] https://drafts.csswg.org/css-color-adjust-1/#color-scheme-meta
<rdar://problem/58388532>
The tests are now at https://wpt.fyi/results/html/semantics/document-metadata/the-meta-element/color-scheme?label=master&label=experimental&aligned&view=subtest&q=color-scheme
The spec changed after I reported this issue. The meta element no longer sets presentation attributes. This is probably a Wontfix.
Resolving as Wontfix per https://bugs.webkit.org/show_bug.cgi?id=205798#c3. (There are still a couple failing tests in https://wpt.fyi/results/html/semantics/document-metadata/the-meta-element/color-scheme/meta-color-scheme-attribute-changes.html but those are unrelated to this issue — and the other failing tests in https://wpt.fyi/results/html/semantics/document-metadata/the-meta-element/color-scheme are fixed by https://github.com/WebKit/WebKit/pull/17817.)