REMs

They’ve been around for a while, but not very popular.  With the rising tide on responsive design, and mobile web usage, it looks like there may be more attention around this concept.

Throw in a css pre-processor and using them becomes much easier.

If you want to read up on the subject, here are 2 links that give you a short but thorough information:

http://www.stubbornella.org/content/2013/07/01/easy-peasy-rem-conversion-with-sass/

http://www.ifdattic.com/2011/10/19/converting-px-to-ems-with-less/

Cheers