> If "View Source" shows <div><div><div>...</div></div></div>, do you really gain much if it shows <custom-element-you've-never-heard-of-with-unknown-semantics><another-custom-element><and-another>...</etc></etc></etc>?
You can extend the semantics of existing elements so you'd actually have <div is="custom-element-with-some-unknown-semantics-but-its-still-mostly-a-div">. Unextended tags are for when nothing in the existing HTML spec mirrors the base semantics you want.
Of course nothing stops people who did bad things before from doing bad things in the future, but it doesn't make tag soup worse.
You can extend the semantics of existing elements so you'd actually have <div is="custom-element-with-some-unknown-semantics-but-its-still-mostly-a-div">. Unextended tags are for when nothing in the existing HTML spec mirrors the base semantics you want.
Of course nothing stops people who did bad things before from doing bad things in the future, but it doesn't make tag soup worse.