I suppose my real question is (and maybe this is a discussion that's been beaten to death and I'm merely ignorant of it): why does the standard require it, if modern technology seems to have obviated the need for it?
Am I too far off the mark with "backwards compatibility" as a scientific wild-ass guess or is it more complicated than that? And if so, with what kind of devices/browsers that would still be in use today?
It is for backwards compatibility, but you're thinking about it in reverse. It's so that modern browsers can also render pages written to older standards correctly as well. They will not use "HTML 5 mode" unless the page explicitly declares that it's HTML 5. Nor should they, because you can't have a valid HTML 5 page without it.
The loud noise you're hearing is the sound of a bunch of old, rusty mechanical gears clanking into place. This explanation makes perfect sense to me now. I hadn't done much serious front-end things since high school in the early 2000's and even then I had no idea what "web standards" were. I just knew how to make pages and display images for a counter-strike 1.6 clan haha.
I suppose my real question is (and maybe this is a discussion that's been beaten to death and I'm merely ignorant of it): why does the standard require it, if modern technology seems to have obviated the need for it?
Am I too far off the mark with "backwards compatibility" as a scientific wild-ass guess or is it more complicated than that? And if so, with what kind of devices/browsers that would still be in use today?