(I'm the maintainer of Blitz)

My experience is that it varies greatly by feature.

Something like border styles, the agent was able to do pretty effectively. Although it still took a few hours iterating on it to get the performance characteristics and edge cases right.

More complex features like hoisting position absolute nodes and other layout features still take days to weeks of iterating to get something usable that isn't so fragile it'll blow up the next time you touch something.