The definition of the safe area inset maximum inset variables says this:

Unlike the safe area inset variables which are dynamic values, the safe area maximum insets are static values that represent the maximum value of their dynamic counterpart...

Though in the definition of the safe area inset variables there is no mention that those are actually dynamic but rather refer to the form of the display. It also says

For rectangular displays, these must all be zero...

indicating that those variables are static.

So the specification of the safe-area-inset-* variables should be changed to clearly state that they are dynamic and what can affect them.

This is also important in regard of the use case covered by the fullscreen-inset-* variables implemented in Safari. I.e. if the safe-area-inset-* variables are not affected by UA-supplied UI like close buttons or picture-in-picture buttons as outlined by @smfr in #11899 (comment), we'd probably need another set of variables that are affected by them.

Sebastian