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Here is the demo file (cf. the first paragraph and the end of the article): https://github.com/gildas-lormeau/Polyglot-HTML-ZIP-PNG/raw/...

This opens a download dialog for me rather that render the html (in firefox on android)

This is done on purpose, so you can rename the file to make sure it's polyglot.

Thanks, on an actual computer it's easy to check :)

For the record, I've just added a link to view the file without downloading it.

A screenshot would help

A screenshot of what? It just looks like a normal web page.