Is this what you're looking for? https://mermaid.js.org/syntax/sequenceDiagram.html

Works fine in VSCode, with the markdown preview mermaid support extension.

That's just a link to the page about sequence diagrams. I know about sequence diagrams - what I don't know how to do is to add a class to a node on there.

Sorry that should have been https://mermaid.js.org/syntax/sequenceDiagram.html#classes-u...

The section where you can add styling by using css classes.

No, that's not what I'm talking about at all. I'm talking about adding a custom css class to a particular node so I can attach behaviors to it with JavaScript.

Ah I see, I guess you're going to be generating the mermaid diagrams? Depending upon how heavyweight you want to go, something like

https://www.npmjs.com/package/vue-mermaid

can work. I've not looked at how it attaches the behavior to the node, there is a way to do it though.