Jeremy Church:

While em is relative to the font-size of its direct or nearest parent, rem is only relative to the html (root) font-size.

Jeremy tends to favor em, because of the ability to control an area of a design. As in, scale the type in that specific area relatively. I have tended to like rem because of the ability to scale type across the entire page easily, but I’ve gotten into issues where that wasn’t good enough control so I could see moving back to em for that reason.

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Confused About REM and EM? is a post from CSS-Tricks