note again that I proposed that strikethrough is --strike through-- and strikeout is >>~~ strikeout ~~<< ( uses >><< to safely allow for striking out trailing spaces.)
This is since they have distinct semantic meaning. Strikeout is more of a critic’s markup Reviewing Markups
Since core spec is not going to be updated in a while. An unofficial extention spec might be handy in http://www.reddit.com/r/LightWeightMarkup/wiki/commonmark/unofficialextensions to keep everyone in the same page.