Wikibase and Wikidata should have clearly defined browser compatibility goals (JS, CSS, DOM), similar to the one defined for MediaWiki at https://www.mediawiki.org/wiki/Compatibility#Browsers

For combination of Platform/Browser/Version, the support level should be defined as one of the following:

1. none/unknown (we don't know, we don't care)
2. reading (possibly no editing at all, but everything must be readable)
3. basic editing (possibly some parts not editable)
4. all parts editable (but maybe not nice)
5. full support (do all the shiny things)