Client side include feature for HTML

[TakayoshiKochi]

Spun off from HTML modules discussion

There are certain amount of interest that including HTML snippet into an HTML document, without using JavaScript. That would be similar to <iframe>, but more lightweight and merged into the same document.

It would work as a naive way to have your header and footer sections defined in one place.

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(Edit by @zcorpan: also see https://github.com/whatwg/html/issues/3681)