In the spec it is mentioned that a blockquote doesn’t require a whitespace between the blockquote character > and the content of the blockquote (example 193).
Firstly, it makes a small inconsistency with Heading ATX that require a space (example 33).
Secondly, we come across a collision with textual smileys. For example, the smiley >_< are not parsed like a smiley but like a blockquote with the current commonMark spec.
This would be a good rule to add to a style guide for CommonMark. You could potentially run an auto-fixer on existing CommonMark documents to enforce the rule.
We should have changed the behavior together with prefixed, ATX headings, which broke, for instance, a lot of readmes on Github anyway.
I believe a compromise would make – or rather: would have made – most sense: a space or tabulator after the “bird track” is optional only if another line marker (#, *, -, +) or fence follows, otherwise whitespace is mandatory.
>- quoted list item
>* quoted list item
>+ quoted list item
> - quoted list item
> - quoted list item
>- quoted list item
>- quoted list item
> - quoted list item
> - quoted list item
> - quoted list item
> - quoted list item
> - quoted nested list item
>- quoted list item
> - quoted nested list item
Fences
> ~~~
> quoted code
> ~~~
> ```
> quoted code
> ```
>~~~
> quoted code starting with literal space?
>~~~
>```
> quoted code starting with literal space?
>```
>~~~
>quoted code starting with literal greater than sign?
>~~~
>```
>quoted code starting with literal greater than sign?
>```
Thematic breaks
>___
>---
>***
Optional blockquote suffix
By the way, I also believe that this should have been complemented by the introduction of an optional suffix < with the same whitespace rule in front of it: