I just tried Focused in Sierra on an document I had been editing before the MacOS upgrade. The document does not appear but the preview does. When will Focused work in Sierra?

John's work was ground breaking and over the years various extensions were built and faster parsers created. I like Fletcher's MultiMarkdown which includes John's original spec plus lots of well thought out extensions. Fletcher has released version 5 just recently and I have built his code on old Macs and Raspberry Pi. His build process includes lots of tests.

Common Mark is interesting but, IMHO, has a long way to go. I will be monitoring it.

Most of the time one doesn't need much more markdown than John's original spec. I use a lot of features that are only available now in MultiMarkdown.

Brett Terpstra wrote a good article on writing better markdown. In it he discusses some issues between different markdown processors with respect to lists. He uses a lot of different markdown editors had he is the author of Marked 2 which is, IMHO, a great app for previewing markdown files.

Another good source is Fetcher Penney's site who maintains the MultiMarkdown processor that Focused seems to use. See the MultiMarkdown documentation for a pretty good description of all the features. I appreciated when he added abbrevs for when I have to write technical papers with lots of acronyms.

I have always found adding a blank line before and after lists, headings, quotes, etc made for easier to read and process markdown.