User doesn't know server seed in advance. User only knows encrypted server seed - for future check if roll was fair. Website doesnt know client seed in advance.
So theres no way the website, server, who you enter that information into and click submit, could possibly get that seed?
Its a webform. You're submitting a variable and clicking send.
Are we just taking their word for it that they would never "peek" at our cards?
The server gives you the hash of the server seed. At that point, the server seed is fixed and they can't know what you will enter as client seed.
Now (after that) you enter your client seed, which can be anything you want, submit it and if the server seed (hash) doesn't change,
there's no way the server could make any adjustments based on your client seed.