Hmm, ok, my apologies, looking at the source code, it seems to only currently support embedding YouTube and Vimeo. It's open source, so i guess someone could fork the source and add more supported platforms, then submit the changes..
Line 140-170 https://github.com/steemit/condenser/blob/master/src/app/components/cards/MarkdownViewer.jsx
Kinda strange to specify vimeo in the else statement, if they hadn't it would arguably embed anything you like, be that vimeo, d- tube or whatever?? As it just creates an iframe..
#confused :/