Seeing this on my iPad and MacOS Safari:

Note the blank background for the top of the header where the menu items are. Same (but even worse!) in Portrait:

Same on Safari on MacOS:

Can't repeat in Chrome nor Firefox on MacOS so it's Safari specific.
I'm guessing maybe part of https://github.com/HTTPArchive/almanac.httparchive.org/pull/492 @catalinred ?
Changing background-size to cover instead of 100% 100% seems to fix it, but does move the image about a bit.
@bazzadp let's make the cover fix even if temporary. If you've tested it and it improves the experience, let's go for it. Great catch, thanks for reporting it.
BTW on Chrome it looks like this:

With my change it looks like this on Safari:

So not quite the same, but better than whats in there now!
Let鈥檚 reopen the issue while @catalinred investigates further.
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BTW on Chrome it looks like this:
With my change it looks like this on Safari:
So not quite the same, but better than whats in there now!