Just noticed that @vitaly 's https://markdown-it.github.io/ demo contains custom containers. Glad to see that it it works well at the very least as a container format. Degrades rather gracefully as well. Great work!
Would be interesting to see how people find containers, and if it causes any problem. Also if it’s possible to use it as a container for latex maths or something like that.
:::: warning ::::
*here be dragons*
:::::::::::::::::
Degrades to :::: warning :::: here be dragons :::::::::::::::::
in common-mark strict mode. Else it uses warning
as the classname to wrap around the *here be dragon*
content.