Not sure whether this ought to be done or not. Certainly abstracts [that's the traditional word for "TL;DR" and it also has the advantage of not being pejorative] would be a useful addition to the web, but I don't know whether a browser plug-in would be the best way to do it.
Off the top of my head, encouraging authors to provide their own is a Good Thing, but they can easily incorporate that into their own websites, so no plug-in is needed. What is really needed is a culture that encourages such things (as, for example, the culture surrounding academic writing has, for decades).
As for off-site abstracts, might including such things in sites like HN (see the comment by sorbus) be an effective way of presenting them? And sites like HN already include the voting.
Off the top of my head, encouraging authors to provide their own is a Good Thing, but they can easily incorporate that into their own websites, so no plug-in is needed. What is really needed is a culture that encourages such things (as, for example, the culture surrounding academic writing has, for decades).
As for off-site abstracts, might including such things in sites like HN (see the comment by sorbus) be an effective way of presenting them? And sites like HN already include the voting.