BYOND/Space Station 13 is built upon this model.

Sprite sheets are png with ztxt blocks with meta/frame info and a list of drawing operations to be done to construct vsprites based on any runtime server side operations done on the sprites.

There is limited client coding via popup Web view windows and a few js apis back to the client but nothing you can build too much off of.

(SS14 brings this model to an open source c# framework called The Robust Engine but has some limitations related to maintainer power tripping over who should be allowed to use their open source project.)

Amazing! Never heard of this byond/ss13/14

Thank you for the reference!