In what way is that not currently possible? All browsers I know of you can edit whatever you want in any page you download

You'll need to do a bit of work to make it the way it used to be. Editing any text on a page, or having your changes save persistently, needs a bit of a... Framework, to keep things together, rather than being the expected mode of interaction.

Sure, I can add a p to the tree. But if I refresh, its gone. I'll probably need plugins to keep my own stylesheets and JS changes around.