[css-images][css-values][css-gcpm] qrcode() for generating QR code images from URLs

Generating QR code images from URLs is more and more common these days. It'd be nice if you could generate QR code images from CSS.

For example, you could generate QR codes for links in a print stylesheet:

@media print {
    ul.citations > li > a::after {
        background-image: qrcode(attr(href));
        ...
    }
}

Or you could generate a QR code for the document itself in a print stylesheet, so someone could find the online version of an article in print (see also #6546):

@media print {
    body {
        background-image: qrcode(url(...));
    }
}

Thought of this while reading a twitter thread between @jyasskin @dauwhe @tabatkins: https://twitter.com/jyasskin/status/1430586358658043909

Review the proposed qrcode() examples and related issue #6546, then trace the CSS Images, CSS Values, and CSS GCPM specifications named in the title. Done would require an agreed design and corresponding specification text, but the issue does not identify files, tests, or a settled implementation approach.