alt-left makes the most sense to me for going back in history, and alt-right for going forward, of course.

slightly off-topic: i had a keyboard (IBM, i think) with "back" and "forward" buttons just above the left and right cursor keys. they just sent alt-left and alt-right signals--no special drivers necessary. it was convenient for 3d shooters before mouse-look became ubiquitous, because i could use those keys for side-stepping.