@kovidgoyal thank you for your reply. I didn't see it before I wrote my reply.
When I look at the Console in Calbre, I see the error:
Quote:
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DevTools failed to parse SourceMap: clbr://internal.sandbox/book/vis.map
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Hmmm. What does that mean?
vis.js is one of the files in my ZIP. But even when I had brought the js files inside my HTML file so it was just one big file, the canvas element still wasn't rendering in Calibre.