This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
:-moz-system-metric(scrollbar-thumb-proportional) CSS pseudo-class will match an element if the computer's user interface uses proportional scrollbar thumbs; that is, the draggable thumb on the scrollbar resizes to indicate the relative size of the visible area of the document.
This selector is intended for use mainly by theme developers.
Not part of any specification.
No compatibility data found. Please contribute data for "css.selectors.-moz-system-metric.scrollbar-thumb-proportional" (depth: 1) to the MDN compatibility data repository.