It ultimately comes down to your personal preference and style.
In my experience, I tend to use:
- REM for font sizes with a px fallback
- EM for paddings and margins
- px, %, vw, and vh for sizing elements
- For media queries, I lean towards using em, but rem and px are also common choices.
Using em for font sizes can cause issues with child elements, so it's generally not recommended.
I find REM to be intriguing for sizing elements, but I've been finding great success with utilizing vw and vh for responsive design in cases where % falls short.
Additionally, the CSS calc function allows for some interesting possibilities,
such as width: calc(100% - 85px);
for specific fixed sizes within your layout.
For further exploration on this topic, check out this recommended read. I hope this information proves helpful to you.