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Note that if you're on iOS, it's possible that the HTML page doesn't work at all because when it's opened from the filesystem, it's displayed by a viewer which doesn't support JS instead of Safari.

It's the "Rennes JS User Group" image that you see in the middle of the HTML page.

I am also on my phone and found it weird that wasn't a single online demo

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/...

A screenshot would help

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

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.