[geofft]

> Sensible defaults are important; if you can guess what users want most of the time, then you should just do that.

That is a good general rule. In the case of DNT, the header was formulated specifically with the intent that the default would be off, regardless of what you expect the user to want, so that turning it on communicates individual user intent. This is a reason to ignore the general rule in this specific case.

A good related example would be license agreements. Most users want to ignore them entirely. Focus groups would indicate skipping them. But if you make a click-through license agreement invisible, while that's a better UX, the agreement is now completely legally invalid. In order for the agreement to be valid, you need the user to have an opportunity to read it (even if focus groups indicate nobody does).

And while you expect 100% of your users to accept the agreement, the default needs to be "No, I do not accept".