I have done some work on Vado in my free time.
Nowadays Haskell really feels like a sweet spot for the niche of toy browsers: fast enough to be practical (unlike scripting languages), with automatic memory management and decent libraries and library management (unlike C/C++). Writing small parsers and CSS handling logic is a breeze. Libraries were also decent: bindings to SDL, Cairo, even Pango; image loading, asynchronous networking/parsing is there. On the other hand, a lack of a GUI framework was quite painful and limiting (I only managed to implement a very crude text input before getting bogged down in thinking about writing a GUI framework).
Haskell really was a secret weapon before Rust and Go, it’s a shame the ecosystem was not on par at that time.