leftmargin and topmargin attributes should apply to both sides #11879
Description
What is the issue with the HTML Standard?
https://html.spec.whatwg.org/#the-page
In Chromium and WebKit, leftmargin sets both margin-left and margin-right. rightmargin is ignored. Similarly, topmargin sets top and bottom margins.
Demo: https://software.hixie.ch/utilities/js/live-dom-viewer/saved/14289
Gecko ran into a web compat issue due to this: https://bugzilla.mozilla.org/show_bug.cgi?id=1997310