If you go to Prefs - Behaviour and find the list labelled 'Use internal viewer for' (right-hand side), you can uncheck the box for ZIP. Then when you use calibre to view the ZIP file it will open in whatever default program your operating system uses for ZIP files (e.g. WinRar, 7-zip, ...). You can then double click on the html file(s) inside and it will open in your default browser rather than the calibre Viewer.
At least, this works in Windows...
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