Describe the bug
In game and review mode some icons are missing and just show an empty square. This appears to have just started today (11/21/2020). I can repro on both desktop (Windows 10 Chrome) and mobile (Android Chrome).
Game mode (see "Zen mode" and "Toggle coordinates"):

Review mode (see "Zen mode", "Toggle coordinates", and "Original game"):

Analyze mode controls (see stone marking buttons):

I noticed the Play icon was gone earlier today too (Android Chrome, MacOS Safari)

Can confirm I too am experiencing this issue with icons for Play, Zen, and Toggle Coordinates not loading.
Windows 10
Brave Browser Version 1.17.73 Chromium: 87.0.4280.67 (Official Build) (64-bit)
Bisecting shows this as the problem commit (upgrading to webpack 5): 00b295ef6aa676ad5264c8f49e9c57e9d365b5b2 (cc @anoek )
PR with fix is out: #1281
Took me longer than I'd like to admit to hunt down as I was convinced it was a webpack issue due to the nature of the offending commit and went down that rabbit hole.
Most helpful comment
Bisecting shows this as the problem commit (upgrading to webpack 5): 00b295ef6aa676ad5264c8f49e9c57e9d365b5b2 (cc @anoek )