I'd say it should offer some form of API. Then you can use anything to read. Such as Emacs.
I thought it would first render the page, then the javascript would take over. It does not seem to be the case, on first load there's only javascript.
Running without JavaScript is a use-case. Not everyone wants to run that insecure stuff on their machine.
The distinction is that JS is a complex and turing-complete language that browsers put a lot of effort into making fast and memory-efficient.
And as a result JS doesn't exactly have the best security record.
2. Even if you assume JavaScript sandboxes are secure, the actual expected behavior of the language is that all code runs in a big global scope. A perfect sandbox would prevent sites from running arbitrary code on your machine, but any JavaScript running on a page effectively has access to all the data on the page. That means every ad, tracker, etc. on the page has access to all the data on the page, including passwords on forms, etc.