The logo currently uses flex layout with float fallback for some old browsers (patch). There is a simpler, compatible way to do this with just display: inline-block which is supported by browsers on our radar (ref).
Admittedly I've over-engineered those rules and I'd like to make up for the oversight.
This reduces the number of CSS rules by about 5, mostly by eliminating the vendor prefixed flex rules.
The patch is a spin-off of T256893: Move the PersonalMenu to the header