Very nice! I was looking for something like this-- A diagramming tool with plain ol' css.

Given the features I think it's a nice step up (flowchart.fun is $3/month) from an Open Source diagram tool like MermaidJS [1].

One suggestion: a link to the diagram-coding docs from the editor.

[1] Here's an example of how MermaidJS styles a flow chart:

https://mermaid.js.org/syntax/flowchart.html#styling-a-node