Is it safe to not provide a backup CSS rule for measurements relative to the Viewport?

Answer 1

Before I answer, I'd like to clarify the term "backup" in this context. Rather than saying backup, it's more common to use the term fallback.

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The answer to your question is like any other answer in web design/dev. It depends.

If your analytics show that the vast majority of your visitors use browsers that support viewport units (vw, vh, vmin and vmax), then the answer is: Yes, it should be safe to not provide fallback rules for viewport properties.

But if on the other hand, you somehow still need to support legacy browsers, than the answers is: No, it isn't safe to not provide fallback rules for viewport properties.

All browsers have been supporting viewport units for a while now, so generally speaking, it's safe to use them without declaring fallbacks.

Up-to-date info on Caniuse.com: https://www.caniuse.com/#feat=viewport-units

Hope this helps.