No, but yes. It happened on multiple sites as soon as I installed 26.0. They were fine before then. I have heard of some web devs being paid $$$ to fix some sites that had this issue.
I haven’t run the websites through a validator. I don’t know if a) the code passes validation generally and b) if it fails, if the validation failures are the cause of this display bug. Frankly I have better things to do with my time than try to fix other folks’ websites which I won’t be getting paid to sort.
The way the websites all broke right when 26.0 dropped points at something changing in Safari. I don’t run many extensions but even disabling my ad blockers the issue is still there. Sometimes it vanishes, something to do with loading and reloading, I can’t work it out.
The Safari flags is interesting – I will have to investigate that. Maybe it’s cache related?