Back and Forward buttons not specified for MouseEvent.buttons

[CendioOssman]

There appears to be a de facto standard for how the back and forward button of mice are mapped to the MouseEvent.buttons field. I would appreciate it if this could be made de jure to avoid future bugs and confusion.

It is nicely specified for MouseEvent.button. But unfortunately, you cannot blindly assume what that means for MouseEvent.buttons as the two attributes already use a differing ordering of buttons.