See https://wpt.fyi/results/css/css-values/calc-z-index-fractions-001.html?label=master&label=experimental&aligned&q=safari%3Afail%20firefox%3Apass%20chrome%3Apass
The code is here: https://searchfox.org/wubkat/rev/94656d3143fad07c748addf564baeec77318f50e/Source/WebCore/css/parser/CSSPropertyParserHelpers.cpp#269
<rdar://problem/114532926>
Pull request: https://github.com/WebKit/WebKit/pull/17113
*** Bug 195026 has been marked as a duplicate of this bug. ***
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/41660
Committed 267339@main (98b6a2103ec0): <https://commits.webkit.org/267339@main> Reviewed commits have been landed. Closing PR #17113 and removing active labels.
*** Bug 263141 has been marked as a duplicate of this bug. ***