People are requesting this more and more. I think it should get an extra setting in official Vivaldi. I don't know about the first versions, I would be interested to see other solutions. Anyway, I put some code together for this some time ago, really not sure if it's the best way to go about it, but it's working.
Normally I'd agree with you, but this is a source editor. Running html found in the wild through an html parser then reading back the result is very highly likely to produce html that doesn't match the original source.
Also while Chromium has a much higher limit some browsers are limited to urls 2083 characters in length (this bookmarklet is currently 1655 characters). Implementing a js based html parser is going to add a lot more code.