CSS Question EM vs % vs PT vs PX

I am trying to create a new CSS within Robohelp 10. In the past we have used pts to identify fonts and indents. What is the current best practice? Should we be using EM or % instead? How does this impact the output both in print and web formats?

Also, I am having trouble applying an updated style to old projects that were recently moved from RH 6-10 and then new CSS was applied. It is not applying consistently.

Thanks for your help!