Bitbucket 5.3 and 5.4 don't render HTML files with .md extension. Up to version 5.2, any .md file seems to render HTML. The problem seems to start after 5.3 version:
Versions prior to 5.3 can render HTML tags in any files (if they have the .md file extension):
- Push any .md extension file with some HTML tags.
- Open the file in Bitbucket Server