Your comments

Then why not use MacVim?

Don't get me wrong, I love vi and I use it daily. But I don't understand this obsession with trying to turn every text editor into a vi-alike.

I would prefer a WebKit sheet in the same window as the document that I can toggle with a keyboard shortcut.

Opening in a browser means the file has to be saved somewhere and it opened in the browser. I then have to clean up manually. At the very least I have to close the browser tab. This sucks for rapid iteration where I make many changes and preview them in succession. Making 20 edits and having each of those sitting in browser tabs is tedious.

I don't seem to have any problem saving with .md.