CSS3 - 知乎 https://www.zhihu.com/topic/19562598/hot

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.

Confused About REM and EM? | CSS-Tricks https://css-tricks.com/confused-rem-em/

There’s more to the CSS rem unit than font sizing | CSS-Tricks https://css-tricks.com/theres-more-to-the-css-rem-unit-than-font-sizing/

 Font sizing with rem - Snook.ca https://snook.ca/archives/html_and_css/font-size-with-rem