Thank you for the reply.
I don't follow why it stopped working all of a sudden, as same links worked earlier. I'm sure I shared links for R docs, and especially R4DS a lot in the past. It worked then. So, it seems that they must have correct tags. Even old FAQ had these things, which seem broken now:
Stack Overflow and Github links seem to work fine even now. Not sure what else. I checked few other forums, for example Manjaro one, and onebox seems to work there fine.
Do you mean that something changed recently in this forum particularly? Or, am I missing something obvious?
(Sorry, but I'm extremely unfamiliar with HTML and related stuff, so tags are not for me 
. Please bear with me on this.)