iPad Pro 1.5 hours web browsing only using 8% battery !!!

The headline made me think you had 8% left! Brightness is the major downfall, if the room is bright I've had 13 hours out of mine. Dim room and it drops to approx. 9 hours.

Surfaces are typical Windows machines, which isn't a bad thing for a lot of tasks, but remember to take a charger. The iPad is a different beast, it is a genuine portable 'all day' device, with a trade off.

 

I think you got this backwards. If the room is bright, generally you need to turn the brightness up to see things, which uses more power meaning less battery life. If the room is dark, you can turn your brightness down and get better battery life.

 

Yes it takes a while for any iPad to drop from 100%. I've got a almost three year Old iPad 4 and after fully charging it after one hour and a half of any non-draining activity (Such as, as you said, web browsing) I will be at approximately 92-95%, so it's definitely possible.
It's also correct what you said on the 100%. I've seen it dropping to 99% after as 'little' as 35-40 mins (45-55 being the norm), also, my record up to now is that it has dropped to 99 after one hour and ten minutes.