CSS rem vs em

rem

• The font-size of the root element (i.e. the html element)
• Using rem for units means using multiples of the root font-size.

em

• The font-size of the of the current element

References

https://css-tricks.com/theres-more-to-the-css-rem-unit-than-font-sizing

https://developer.mozilla.org/en-US/docs/Web/CSS/length

http://stackoverflow.com/questions/13941275/rem-in-css-in-what-way-it-differs-from-em/32262283#32262283