First frame after insertion FOUC

[dy]

That doesn't seem to be mentioned in readme, but the callback skips at least one frame after the element is inserted to DOM. So for example if there's initializer function for a rule, that adds some styles or content to an element that is added to DOM, the element still appears in DOM unstyled for a single frame, then the rule is hit. That creates FOUC.
I guess that can be prevented by hiding element for that frame in some way...
That is the case for spect. selector-observer doesn't have that issue.

[naugtur]

Can you share a reproduction case? I'm not sure I understand the problem.

[Erudition]

set timeout lower than the duration of one frame, maybe? see the config