Using phantomjs. For medium it just grabs an image on all renders. I considered making it not show the gif at all in the index for sites on medium for now... but I'll try to get that debugged soonish by someone familiar with phantomjs.
A lot of sites provide screenshots. No legal trouble is likely there (unless you're attempting to reproduce entire sites that way). The text can be another matter depending on how much you're displaying / reproducing.
Very nice idea otherwise! I wonder if you could get in (legal) trouble for providing the screenshots/texts.