This is a weird bug, check the following test case: https://codepen.io/cl0p3z/pen/QWjKYWm?editors=1111 On the above test if you comment the first line on the JS tab. The one with the console.log("Target size before resize is "...) then the test will pass. That doesn't make sense since its a simple console.log() line. It seems that checking the size of the target element before doing the resize affects the end result (the size after doing the resize). This bug is causing the WPT test https://wpt.live/css/css-sizing/percentage-height-in-flexbox.html to fail (the second test)
Safari Technology Preview 197 shows: "Target size before resize is 50x50" "Target size after resize is 100x100" and also WPT passes as well > https://wpt.live/css/css-sizing/percentage-height-in-flexbox.html