IceBergyn I guess I'm not sure (I tried lots of exports). I eventually tried to settle on the most "compatible" and oldest set of everything. Though please do outline this compatibility mode for me as I very well may have missed it.
In any case, shooting for the "most compatible" meant I migrated back to 3.5 from 4, I decided to export WebGL 1.0 (GLES2) because the documentation said "WebGL 1.0 is the recommended option if you want your project to be supported on all browsers with the best performance." I used the "Regular" export type as "is the most compatible across browsers, will not support threads, nor GDNative" (though I did try the other options and they also did not work). My memory is that there were different compression options (for web, desktop, etc). and that I tried them all to no avail (though perhaps not every combination of them all).
I feel like I must be missing something foundational and trivial, because my understanding is that Godot 3.x very much can have a simple 3d game running in most browsers, but I'm so far unable to make this occur.
Any additional insights appreciated, and thanks all for your help on this!