Unfortunately the idiots at Chrome as well as their counterparts at Firefox broke their existing WEBGL GL_EXT_draw_buffers implementations through a non backward compatible change (championed by the KhronosGroup) that they rolled out in the second half of 2019.
In the past years I repeatedly had to fix my old pages due to Google/Mozilla's pathethic "green bananas software engineering" approach. I've had enough and from now on I will no longer maintain this page just to please the whims of those irresponsible idiots.
I suggest you get yourself a respective portable version of one of the older browser versions that still work correctly (Chromium 75, Firefox 66.0). Click outside of this box to continue.