You can easily write a bookmarklet which you can tap that would replace any viewport meta tag with a viewport tag that allowed zooming, but as has been mentione

You can easily write a bookmarklet which you can tap that would replace any viewport meta tag with a viewport tag that allowed zooming, but as has been mentioned in other thread where this topic comes up - almost invariably the mobile browser software _already has_ a convenient user override built in. Find it and toggle it.