I would like to add to this that this is more so a problem on mobile (i.e. Android) when using privacy hardened browsers such as Fennec or Mull (and probably much more).
On both of those browsers, there isn't even an easy option to enable WebGL (you'll have to skim through about:config) and on both, it's disabled by default.
Also, it took me some time to find this thread and finally realise what's causing the issue.
You should at least add a banner to maps that clearly tells the user what's wrong.
I.e.: "Maps failed to load because you're using a browser that either doesn't support WebGL or has WebGL disabled".
Currently, the UX is just a blank page. And that just sucks 😛