If for some reason WebGL is not being enabled in Safari on your Mac it might be due to a security setting. The fix is to enable WebGL in Safari, but this must be done via a terminal. On the macs that worked , they had no custom prefs set, so WebGL is enabled by default so this might not be an issue on your specific machine.
How to fix:
At a command line:
How to see the error:
defaults read com.apple.Safari | grep -i webgl
If webGL is not enabled, you see the following in the output :
"WebKitPreferences.webGLEnabled" = 0;
To enable WebGL on the failing system, you need to do this:
defaults write com.apple.Safari WebKitPreferences.webGLEnabled 1
Also, even with WebGL enabled, to get the preview to show after adding a block/controller, you may need to refresh the page – before or after the first block is loaded.