which seems to seek only the value "checked"

"checked" is the value that HTML uses. Well, sort of, most of the time. The original specs of HTML are rather vague, and the modern ones backwards compatible. And there is the thing with XHTML, which needs to be valid XML as well as valid HTML.

So a pre-checked checkbox can be specified like this in HTML

<input type="checkbox" checked>
<input type="checkbox" checked="checked">
<input type="checkbox" checked="">
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But in the stricter XHTML spec, only this one seems to be valid:

<input type="checkbox" checked="checked">
[download]

I dimly remember that the return value in a form for a checkbox is also a bit strange, i think it's "on" or something similar. Don't quote me on that, i never use those damn things in my web framework directly anymore sine i wrote a touch-friendly replacement (that also accepts custom values for the "on" and "off" states) about 15 years ago...