That’s a good point. CommonMark is supposed to be all things for all people.
Someday…
But realistically, that day will probably never come. The community is so full of different opinions about the spec that the spec will likely never get finalized.
I still think it’s a good starting point, and eventually it will supplant MultiMarkdown and GFB both. But by my own accord: that day is not today 
Is it heresy to suggest Drafts embed HTML e.g tags round “struckthrough” text?
True Story… CommonMark
I’d like to see strikethrough.
I also don’t know how this addition effects markdown as a whole. I’m looking for an agile system that works on most apps. Portability…
It would not be terribly hard to do post Markdown modifications in Script if you want to add support for some special attributes. I may add additional Markdown engines at some point, but for now it’s MultiMarkdown and strikethrough is not available in it.
It is if you support HTML strikethru.
Preview not appearing for me either, in Mac or iOS.
Old thread, but to summarize:
This ~~strikethrough~~ style is specific to the GitHub Flavored version of Markdown. It is not part of Markdown proper, and not part of MultiMarkdown.
If you want to use it, use “GitHub Markdown” as the syntax highlighting for your drafts (selectable by draft, and can be set as the default for new drafts), and use change your Markdown Settings to use GitHub Markdown instead of MultiMarkdown.