Both EM and REM are flexible, scalable units which are translated by the browser into pixel values, depending on the font size settings in your design. REM is dictated by the set root (HTML document) size.
- Pixels(px) are much more precise and should be rarely used when you want to have a fluid design, unless you want to make a bunch of changes in your media queries.
- The default size, if one has not been defined, is 16px — so 1em/rem = 16px.