The current downside with netsurf is that it doesn't do HTML5 yet.
But then by the looks of things the other markdown renderers don't do that either, excepting typora.
At least, I suspect that that is the reason why the others keep breaking on a direct link to an mp4.
Just tried out some stuff with "netsurf" and I must say that it's extremely lightweight and renders quite nicely. At least it doesn't break on the link ..... it simple doesn't show it.
Using that as a sandbox might just be the solution. 
