This one falls into the “reminding myself how to do the workaround” category, but maybe it will be helpful to someone else, too.
The WordPress media player has some nice buttons for play, volume, full-screen, and so on. But no matter what I did, I could not see those buttons in any browser. They were there, and could be clicked, but were completely invisible.
The problem is that those buttons are produced as .svg files (scalable vector graphics) and some servers have trouble associating mime-types with those. So the server won’t serve them up (or it serves them as plain text, or something, which they’re not, so they’re not visible), no matter what browser you use.
There are various htaccess tricks to register the mime-type, but I couldn’t get them to work.
However, those same buttons also exist right in the same directory as .png files, which the server is fine with, so I just went to /wp-includes/js/mediaelementplayer.min.css
(UPDATE: It’s now /wp-includes/js/mediaelement/mediaelementplayer.min.css )
and replaced every reference to “svg” with “png.” That did it (had to clear my cache in safari to see them there).
It’s ugly to edit core css like this, and will break every time I upgrade, most likely, so I’ll have to do it again, so that’s why I’m noting it here!