But browsers already take care of this. They collapse all the spaces in markup for non-pre-formatted blocks; they render block/inline elements accordingly; inse

But browsers already take care of this. They collapse all the spaces in markup for non-pre-formatted blocks; they render block/inline elements accordingly; insert newlines where needed, etc. And then, once you select some text from the page — there's your "plain text" representation. We probably can't avoid differences among browsers (simply due to the vast nature of HTML — form controls, media content, generated content, new elements like <template>, non-standard extensions, etc.) but in context of a single browser, the logic already exists beneath. It's just not exposed conveniently.