Viewports are hard. Don't let anyone tell you they aren't. Although desktop browsers have only one, and it works pretty intuitively, mobile and tablet browsers have three. And sometimes they don't work intuitively - or at all.
What are the three viewports? (Spoiler: layout, visual, and ideal.) Why do we need all three? Why does responsive design work? (Not how. Why.) What happens when you set the meta viewport? How do browsers go wrong? (Spoiler: in plenty of innovative ways.)
And what about resolution, or DPR? (Spoiler: it is not what you think it is.)
In this session PPK, who spent more time on the mobile viewport than anyone not working for a browser vendor, answers these questions and more.