Tuleap uses michelf/php-markdown  to render Markdown which is nice but it is a specific Markdown implementation and Markdown does not have a specification (at least not one without too much ambiguities). This lead to different behaviors between different Markdown consumers: a same content can looks fine on one and be completely broken on another.
In order to solve that CommonMark  has been developed  and is now used (with some extensions) by large users like GitHub  and GitLab . It would be nice to use a CommonMark implementation in Tuleap as well (yeah no more broken README.md when mirroring repositories o/).
There is a well maintained and high quality library in PHP  to render CommonMark content. A nice side effect is that this library can easily be extended so it would be possible for us to do smart things when rendering a Markdown file in a Git repositories like having the capability to have links to files in the repo or to display images located in the repository.