About Mermaid is supported out-of-the-box by many applications, including:
- Jetbrains IDEs (IntelliJ, etc.) through the built-in Markdown Plugin (see diagrams);
- Visual Studio code;
- gitlab.com renders mermaid diagrams in and md file;
See mermaid-js#integrations for a full list.
GitHub does not support it for now, but there are many browser extensions available that will automatically detect and render the diagrams in md files. I personally use mermaid-extension.