Since when was this OK:

<input type="checkbox" checked>

in particular the checked being like that rather than in key="value" format?

I've never come across that until just recently in starting to learn HTML5 in detail. I've always used XHTML in the past. Is that why
<input type="checkbox" checked> is so alien to me? Was that kind of thing OK in all previous versions of HTML (not XHTML)? I suspect that's the case.