CommonMark says that the specification of Markdown’s syntax is ambiguous and has resulted in many different Markdown implementations which have all diverged from each other. This resulted in the same markdown document being rendered in surprisingly different ways in different systems.

I know from experience that this was true, before Markdown became more standardized. It looks like CommonMark played a significant role in that, since it is used in the following places where I also recall seeing a more standard treatment of Markdown over the years:

• GitHub
• GitLab
• Reddit
• Stack Overflow / Stack Exchange

Source: commonmark.org