As mentioned by Clement on Mickey's blog post, Emacs 27.1 can use Cairo to take SVG screenshots of itself. The result is, of course, resizeable without aliasing, and looks great in a Web browser.
(defun screenshot-svg ()
"Save a screenshot of the current frame as an SVG image.
Saves to a temp file and puts the filename in the kill ring."
(let* ((filename (make-temp-file "Emacs" nil ".svg"))
(data (x-export-frames nil 'svg)))