The WPT at css/css-transitions/pseudo-elements-002.html is a failure. It's a simple test, which looks like this: <style> #inner::before { content: "This text should transition from red to green."; height: 100px; transition: height steps(2, start) 1s; } .flex #inner::before { height: 300px; } .flex { display: flex } </style> <div id="outer"> <div id="inner"></div> </div> <script> test(() => { assert_equals(getComputedStyle(inner, "::before").height, "100px"); outer.className = "flex"; assert_equals(getComputedStyle(inner, "::before").height, "200px"); }, "Check that transitions run on a pseudo element whose ancestor changes display type."); </script> From what I can see via logging, we: 1. create a PseudoElement as we query the ::before computed style the first time 2. set the CSS "flex" class 3. return the ::before computed style for the second failed assertion 4. create the "height" transition from 100px to 300px (too late, the previous style check did not account for it) 4. destroy the PseudoElement, terminating the transition 5. create a new PseudoElement 6. eventually consider running a transition again but the before and after styles for height are both 300px so do nothing Something is not being invalidated correctly between setting the "flex" CSS class and querying the computed style. We should have created the transition while the computed style was queried, but we didn't.