Line9 takes Mermaid source and lays it out with its own engine rather than the reference renderer, which is the entire point of the tool. The editor keeps the source on one side and the render on the other, marks the diagram as out of date when the two drift apart, and lets you switch themes and adjust orientation, spacing, and node size without touching a line of source. A finished diagram can be published from the same screen, and a report link sits next to each render for layouts that come out wrong. The site states that the online editor is free and needs no account, with a Pro tier and sign-in above that.
Like this product?