On MediaWiki.org (1.23wmf9), MW thinks that the appropriate MIME type for SVGs embedded in CSS as data: URIs is application/xml instead of image/svg+xml, causing them not to show up in any browser. It works for me locally.
Version: 1.23.0
Severity: major
URL: https://www.mediawiki.org/wiki/VisualEditor/Basic_example_worksheet#Links