Definitely something I didn't know I wanted! Haven't tried it but it looks great. I think you should explicitly say in the description that users will save on cellular data/roaming fees as a main use case.
Brilliant idea. I had this idea myself but I love your execution. It's awesome! My idea was to 'provide a service that makes websites as light as possible (while still user friendly)'
Use Ghostery, NoScript, or uBlock to disable Javascript/downloading certain files. Then add a rule for "*.css" to block any CSS being applied.
Optionally, without using any addons, disable Javascript under your Firefox settings. Then press press Alt to bring up your Menu Bar and under "View" go to "Page Style" and select "No Style". (Or "LeftAlt, Shift+V, Shift+Y, Shift+N")