I'm not sure if you're thinking it would be web server dependent, but NGINX is the bigger share of the market. (Ideally it wouldn't matter what the web server is though).
I'm not sure I understand the pricing plan - limiting on pages tested seems like a bad way to get customers. I would plan on testing quite a few pages per site and testing those after each commit.
This is exactly what I wanted to know. Last time I tried it it was nice, but without the ability to read PSDs correctly (and not misinterpret things to be all over the place) it's quite useless to me.