That's OK!
I'm glad my question helped others making their game better.
I'm planning on using Monogatari as the game engine so I would have only a web version, what I was concerned about was wether the player's devices (especially older phones) would support the modern filters and effects that you can use with CSS, so a prerendered sprite applying those effects should fix the problem of having a tragic scenario like:
"Now I'm going to reveal the face of the villain... But it was visible from the start because the filters didn't work!"
In that situation I think I'm still outside of the rules though, so I already came up with a way to avoid the issue altogether.