[eyelidlessness]

> The "bad input" is, arguably, no longer bad input.

What? Yes it is! Defined behavior for invalid markup doesn't make that markup valid.

HTML5 doesn't refuse to accept anything that HTML 4 accepted. Defining behavior for invalid markup does not even impact "be liberal in what you accept", the scope of what is accepted hasn't changed. It affects "be conservative in what you send", in particular it more closely matches that half of the principle.

[gsnedders]

> HTML5 doesn't refuse to accept anything that HTML 4 accepted.

It does. It doesn't accept NET syntax, i.e., `p/This is contents of a p elements/`. (No browser ever supported this, but because HTML 4 is defined to be an SGML application and it's DTD allows NET syntax to be used, it is theoretically conforming HTML 4.)

[eyelidlessness]

Ah good point, thanks for the correction.

[gsnedders]

(There's also another load of SGML bits of syntax that browsers have never supported which HTML5 doesn't support. Indeed, HTML 4 has a whole section of such things: http://www.w3.org/TR/html4/appendix/notes.html#h-B.3.3)