Viewport units are things like 10vw (10% of the viewport width) or 2vh (2% of the viewport height). Useful sometimes, as they are always relative to the viewport. No cascading or influence by parent elements.
There is also vmin, which is the lesser of vw and vh, and vmax, which is the greater. These values can be used for anything that takes a length value, just like px or em or % or whatever.
Let's take a look at a tiny design pattern that makes use of