When we introduced special compression rules for replaced elements in #765 (see #765 (comment) ), we resolved to make max-width percentages apply the same compression to semi-replaced elements (e.g. INPUT) as width percentages (since they're consistent for replaced elements such as IMG). But this hasn't been implemented yet, so opening this bug to track implementation and Web-compat.

Mozilla bug seems to be https://bugzilla.mozilla.org/show_bug.cgi?id=1388840 (@dbaron had patches but hasn't posted them?)