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I was under the impression mermaid is a wrapper around graphviz dot.

No, it's an independent, JS-only implementation, and it's way, way less featureful.

I believe it uses an cross compiled version of dot in js

I don't see the infrastructure for that, rather it seems to be relying on d3 and DAG renderers targeting it https://github.com/mermaid-js/mermaid/blob/mermaid%4011.6.0/...