First off, if your code don't have bugs, then that is preferable! In my experience with front web dev the issue has always been different (old) devices. Testing could be done by browser macros that simulate input, like mouse movement, click on x, y, z, etc. Then take screen-shots to see if something is off. You can also make your widgets throw errors (1) and call home via XMLHttpRequest if an error is detected.

1) http://www.webtigerteam.com/johan/en/blog/error_driven_devel...