cc @chrishtr @khushalsagar

Gecko has a property similar to overflow-clip-margin called overflow-clip-box (actually a pair of properties, overflow-clip-box-{block,inline}), which we use to implement some of the form control magic that <input> and <textarea> need for compat (like clipping to the padding box in one edge but to the content-box in another).

It seems like it'd be nice if you could specify the margin in a per-axis basis? We can support the extension internally to remove the Gecko-internal property, perhaps, but if we need it maybe authors need it as well.

If we do this I don't have a strong opinion on whether we should make overflow-clip-margin a shorthand or two different longhands.