The memory consumption, especially the semi-non-determinism relating to lazy evaluation, was the first thing that popped into my mind. But, as I said in a comme

The memory consumption, especially the semi-non-determinism relating to lazy evaluation, was the first thing that popped into my mind. But, as I said in a comment below this, I think that Haskell as the common case language, with some FFI accessible code would probably fit the bill. I can not think of any large Haskell projects in the application space where browsers sit, but it doesn’t mean it couldn’t be done.