There are a lot of markdown-alike implementations around that have slightly divergent behaviours (eg: tables, wierd separators, non-html-alignment, non-html-colours, etc.
For example, some implementations do tables that translate this:
|| table cell || table cell ||
<table><tr><td>table cell</td></tr><tr><td>table cell</td></tr></table>
IMHO, we should have tests with the above pseudo-commonmark, and check that’s it’s not parsed or interpreted differently, so as to be sure that the implementation does not diverge from the spec (otherwise, different implentations will result in different output).