My personal take is that markdown is only good when you can customize it to your needs and situation. There are a million markdown flavours, because everyone who ever implements it decides to add their own extensions to it to suit their situation. There have been attempts at creating a single unified standard, but these miss the point: if we wanted a single shared syntax, we could use html directly; the advantage of markdown(s) is that it lets you create something that looks clean and simple as plain text, while turning into a nicely marked-up document. A "standard" markdown would need to have a "standard" extension mecahnism (or none at all...) which would inevitably look like ass for most use cases.

I nowadays usually just write html directly for my personal documents, because I spent long enough messing around with markdown parsers trying to get them to act how I want. But for a website commenting system (eg), it makes sense to spend some time making the formatting system nice to use, which involves customizing your flavour of markdown. I don't think web browsers can or should try to do a good job of this; it requires to many specialized demands for them to be able to create a generic solution. If you want to write in markdown, its easy enough nowadays to format it on the server, or client side with a small js snippet.