The example CSS in

https://developer.chrome.com/extensions/i18n

uses

padding-__MSG_@@bidi_start_edge__: 0;
padding-__MSG_@@bidi_end_edge__: 1.5em;

but better choices would be

-mozilla-padding-start: 0;
-mozilla-padding-end: 1.5em;


(It also has a body direction that Devlin thinks may not be needed; I still need to investigate that.)