I created this web part to add Mermaid diagram support in SharePoint Online as part of my tutorial series. While working on it, I found this discussion thread: Displaying Mermaid Diagrams in SharePoint Online Markdown Web Parts, which highlights the need for a proper solution.
Open this sample in the gallery (popup panel): Open in gallery
Source: https://github.com/pnp/sp-dev-fx-webparts/tree/main/samples/js-mermaid-diagram