How to write css fallbacks for vh vw

Question

Can anyone explain how fallbacks work in CSS? I am trying to set it up for vh and vw and clearly I am not getting it...

Here is my attempted solution, which does not

Answer 1

Place your fall back above you other values. If the overriding values dont work on a browser, the first value is used.

 height: 41px;  /* The Fallback */
height: 5.2vh;