Created attachment 287241 [details] Test case to reproduce the issue. The CSS Box Alignment specification defines a new syntax for align-self/align-items, justify-self/justify-items and align-content/justify-content CSS properties. This new syntax include new values, some of them not valid following the old syntax defined in the CSS Flexible box specification for some of these properties. Due to the patch landed for fixing bug #151591 we are now sure that the parsing logic for the new syntax is only used when the CSS Grid Layout is enabled, since this new layout model defines its alignment procedures based on the mentioned CSS properties. So, the case is that for some properties, like align-items in the attached test case, the initial value is parsed as invalid.