I apologize for the delay, but thanks to Andrew's excellent replayer, this is now finally enabled on the main site. The basic idea is: Indent your PGN by four spaces (you can use the editors "Preformatted text" button or Ctrl-K), and we'll display the board in its place.
It also works in the editor preview, so you can play around and try it out without having to submit anything.
I'll describe the details below; note that this is not final and is very much up for discussion. You are the ones who know best what makes sense, so please be sure to give your feedback, and we'll try to adjust things for you to be as comfortable as possible for you.
What kind of markup is (currently) converted into a player?
An indented block containing just a FEN string will be displayed as a static board without move controls:
r1br2k1/pp3ppp/2n2n2/1Bb3B1/8/2p2N2/PPP2PPP/R4RK1 w - - 1 13
looks like this:
A FEN string followed by moves will be displayed with move controls. Example:
6k1/1P3p2/2p3pb/2q4p/2rpPP2/3Q2PP/8/5RK1 b - - 0 2
1... Rb4 2. Rb1 Qb6 3.Rxb4 Qxb4 4. Qa6 Qxb7
is displayed as
The FEN string can also be included in a
[FEN "..."] PGN header instead.
What kind of markup is (currently) not converted into a player?
An indented block containing only moves is not converted. The reason I decided to go this way is that there are just too many indented blocks on chess.se that contain moves in one way or the other; often just one or two, and often not even based on the initial board position. Displaying all those as a player widget may a) often look wrong, and b) cause an overwhelming amount of players on some question pages.
So you have to be explicit about wanting a replayer widget by providing a FEN string. If your moves start from the initial position, as a shortcut you can just provide an empty FEN string in a PGN header, like this:
1. e4 e5 2. Nf3 Nc6 3. d4 exd4
which turns into this:
Where to go now?
This initial version is a starting point for discussion. Please let me know what makes sense, what doesn't, what could be improved, etc. (Almost) nothing is set in stone yet.
One thing I could imagine to be nice, but am not sure on how to express it in markup, is the idea of "continuation". For example, if the question contains a few moves and asks "what should I do now?", then an answerer could be able to indicate "continue from the situation in the question" and add their additional moves, without being forced to copy & paste from the question.