About that Opera setting. It is not found in the preferences anymore from what I can see. But we have had it since the earliest versions in INI files and such.
I just wrote something about this over at MSFN. If you want this option in Opera you can go to
Tools > Appearance > Buttons > Preferences and add a dropdown box to any toolbar like I did ( it is very convenient ). This is what mine looks like ...

- 8ovkspO.jpg (18.51 KiB) Viewed 10401 times
... To use it you just click on it to change the identity and then press F5 to refresh the page ( it re-requests it from the server using the new identity so the server then returns a page based on that identity if the page author used browser sniffing to send different pages ).
That was the whole thing here. Having it set to "
Internet Explorer" screwed up both YouTube and Google Images.
If you're using Opera, after adding this option you can experiment at will. I still don't understand the reason for the radically different pages though.