Let's say 50% (or even 30%). How much more code would have been needed to correctly render most of the rest? I strongly doubt it's 200 MiB, probably much closer

Let's say 50% (or even 30%). How much more code would have been needed to correctly render most of the rest? I strongly doubt it's 200 MiB, probably much closer to w3m's size. HTML email is in practice a large subst of HTML4+a tiny bit of CSS.