So what is User-Agent spoofing? Well, let’s say someone decides that he’ll only allow blondes into an event. Depending on how its done, UA spoofing can be like wearing a blonde wig, or it can be like a brunette wearing a badge that says “Blonde.”

For several years, Opera has done the latter, basically wearing a badge that says “I’m Internet Explorer (wink, wink).” The sites with oversimplistic detection are fooled, but anyone paying attention can tell that it’s Opera.

The next question: Why is it even an issue? Well, web developers want to make sure that visitors will actually be able to see the site as intended, but it’s historically been easier to look for the browser’s name and version than figure out exactly what it can do. So developers often do the equivalent of asking someone whether they can speak French by asking them whether they live in France. You’ll get French speakers, but you’ll also block people from Quebec or Haiti, bilinguals, etc.

These days it’s recommended to check for capabilities, not to check the name of the browser and see if it’s on the approved list. It’s not always possible, since every browser has its own quirks, but it produces better results—and blocks fewer people who might otherwise be able to visit your website.