Youtube supports mpeg2ts container format with H.264/AAC media for HTTP live streaming ... http://www.youtube.com/watch?v=hnslRRh2wcA ... this URL generates HLS stream.
On checking the youtube-dl output found following:
format code extension resolution note
151 mp4 72p HLS (worst)
132 mp4 240p HLS
92 mp4 240p HLS
93 mp4 360p HLS (best)
even though extension says mp4 ... the actual container format used when used through chrome browser on windows is mpeg2ts.