It's already desktop mode. As I remember, in the 2007 introduction of the first iPhone, Steve Jobs actually showed that the iPhone could display the full New York Times website.
It's the websites themselves that give you a mobile version that cannot do anything. Some of the worst offenders even detect your screen resolution to give you a handicapped version of their so called website regardless of the User Agent string (i.e. the identifier of what browser you are using).
These stupid website developers just have to stop doing this.
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Downloaded Puffin again, just to delete it right after opening. It doesn't even support the iPhone X screen and the last update came one year ago.