Embedded Gists don't support loading Mermaid charts

[chrisirhc]

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I love that Gists support Mermaid charts, but it seems that the rendering breaks when the chart is embedded on a web page or Github pages.

When embedding a Gist that contains a Mermaid chart, like the one here: https://gist.github.com/chrisirhc/626e485018984e1781c44ec545ed9701 ,

The page ends up looking like this instead of the expected chart:

Demo page: https://chrisirhc.github.io/embed-gist-mermaid-repro/
Demo repo: https://github.com/chrisirhc/embed-gist-mermaid-repro/

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