Actually this happens on Chrome as well with font size set to 80% in desktop mode. So normally it is a non-compat. I think "Automatic font sizing" should not be the default. It affects many users when not using a normal font size on their phone settings. Many of the reports regarding layout on different sites that we do not reproduce might be related to this issue.
Bug 1948221 Comment 3 Edit History
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Actually this happens on Chrome as well with font size set to 80% in desktop mode. So normally it is a non-compat. I think "Automatic font sizing" should not be the default. It affects many users when not using a normal font size on their phone settings. Many of the reports regarding layout on different sites that we do not reproduce might be related to this issue. The users might check on Chrome and say it does not reproduce there but thats because Chrome uses default zoom to 100% for text and images when in reality it reproduces if the size is changed.