On wayland the xbindkeys and xdotool will not work, but you could try from
like this instead:
"xte 'keydown Alt_L' 'key Left' 'keyup Alt_L'"
Also, for me
is pointless because you already have the keyup command … just make it with b:8 and b:9
Probably best for you would be to use AUR (en) - wayland-mouse-mapper-git in your wayland session.
Or even better this http://aur.archlinux.org/packages/key-mapper-git