The behavior of the appearance property is being changed, as shown by the following spec/WPT links: https://github.com/whatwg/html/pull/7004 https://github.com/w3c/csswg-drafts/pull/6537 https://github.com/web-platform-tests/wpt/pull/30267 https://wpt.fyi/results/css/css-ui?label=pr_head&max-count=1&pr=30267
The biggest change as compared to how WebKit currently behaves (and as Chromium and Gecko behaved a few years ago) is that not every element can have any appearance. A <div> can only have the "none" appearance. A checkbox can either be "none" or "checkbox" kind of widget (the latter is equivalent to the 'auto' keyword). And the spec now defines how various author properties like 'background-color' cause the kind of widget to switch to "none", which wasn't interoperable.
<rdar://problem/85826878>