Sizing with rem

Usually font sizing is done with either the px or the em unit. Unlike the px unit, em is relative to the font-size inherited by the element. This is great and has a lot of advantages, however things can get messy as you nest your elements. This brings us to the rem unit, which is equal to the computed value of font-size on the root element. Sounds like the magic solution and in many ways it actually is. Browser support is very good with IE9+.

Resources

An overview of CSS selectors