Dragonfly[0], Presto's old dev tools, was essentially a web app (and, IIRC, although cached was loaded as if any other web app), though it had access to some APIs normal websites did not, to be able to communicate with the browser through the Scope protocol[1].

[0]: https://github.com/operasoftware/dragonfly

[1]: https://operasoftware.github.io/scope-interface/