ASS tags that a subset of ASS for web usage should support at minimum (in my opinion):

Style-related
-------------
Font name: \fn
Font size: \fs
Font scale: \fscx \fscy
Font border size & shadow depth: \bord \shad
Font fill color & alpha: \1c \1a
Font secondary fill color & alpha: \2c \2a
Border color & alpha: \3c \3a
Shadow color & alpha: \4c \4a

Position-related
----------------
General subtitle position: \an[1-9] (see numpad for positioning on screen)
Exact pixel position: \pos(x,y)