This task can be closed as it has been solved in https://phabricator.wikimedia.org/T180890

Sorry, Chrome is working indeed. I also tested Windows 10 / latest Edge. As expected, same thing.

The styles don't get applied in Windows 10 / latest Chrome.

@Esanders I could test on Windows. However, it doesn't look like VisualEditor is installed on the Patch demo.
https://patchdemo.wmflabs.org/wikis/f58002e188/w/index.php?title=Special:Version

I tried the suggestion from @matmarex and added the following to MediaWiki:Common.css as a test scenario:

It would be good if this could be addressed somehow.
In Windows 10, 3 out of 4 high contrast settings use black background. Would it be feasible to replace the background svg-images with font icons so they don't get "lost" when users need the high contrast setting?`

I agree. When a user types a space in the alt-field, it should be treated as if the field was empty. Then the html output would be the standard fallback of alt="imagename.jpg". Normally, this is not WCAG-conform. But in wikis, there is usually an additional image description which often contains the info that you would expect in the alt attribute.