In theory this is what css { font: caption } should do. There are other font shorthand’s for menu text, icon labels, dialog box text, etc. that should clue you

In theory this is what css { font: caption } should do. There are other font shorthand’s for menu text, icon labels, dialog box text, etc. that should clue you in to how a particular system UI is scaled, if browsers are doing it right. (Of course they also might not because of fingerprinting risk) Note that this is different from { font-family : system-ui } which doesn’t include size information. Caniuse sadly doesn’t have much info on font: caption support.