Created attachment 468349 [details] CodePen with the bug reproduced I noticed a bug in Safari (regular and TP) in how it handles mixed units for round(), mod() and rem() that include a %, but only when it does include it for one value (only A or B, not both as in the above tests). First reduced in this CodePen: https://codepen.io/kizu/pen/BaMaYwE?editors=1100 (all elements should have the same width with the green part visible on the right), decided to submit it as my first WPT test: https://github.com/web-platform-tests/wpt/pull/42676 (merged) Can be seen as failing here: https://wpt.fyi/results/css/css-values/round-mod-rem-computed.html?label=experimental&label=master&aligned&q=css%2Fcss-values%2Fround-mod-rem-computed.html Current workaround: add 0% to the other part so both contain px and %. Adding the saved CodePen html as an attachment as well.
<rdar://problem/117554926>
This is fixed in ToT (was fixed as part of the calc() re-write - https://bugs.webkit.org/show_bug.cgi?id=278244)