[phponpcp]

The point that they didn't raise is that scrollbars effect the size and position of things in the browser. Try setting 100vw on an element when you have obtrusive scrollbars, the result will be a page which now has horizontal scrolling, leading to terrible UX.

[chrismorgan]

That’s because the vw and vh units are obtuse, being the full viewport dimensions inclusive of document element scrollbars, and there’s no workaround for that. (There used to be in Firefox, but when the dust settled and all the browsers decided to iron out their inconsistencies, they dropped Firefox’s behaviour as potentially surprising, despite it being the only sane thing out there, and haven’t talked much about making a unit that’s actually useful for this effect.) The effect of this is that the vw and vh units are pretty close to never the right thing. I have before used CSS Custom Properties to achieve a sane result, putting `:root { --scrollbar-width: 17px }` into the document based on a JavaScript calculation, with `--vw: calc(1vw - 0.01 * var(--scrollbar-width))`, so that then you can do `calc(50 * var(--vw))` and it’s like 50vw but sane. But this technique requires JavaScript, so I don’t consider it acceptable in all places.