When doing flexible layouts, there is no way around using relative sizes, so that fonts and elements resize accordingly.

CURRENT RELATIVE UNITS

EM
Relative to the font-size of the element. The information comes from the browser preset.
REM (not supported in older browsers, like IE 8)
Relative to font-size of the root element
PERCENT
Relative to the container

CALCULATION

If not set differently, 1 em equals 16 Pixel in most browsers, which gives us a calculation basis.

So 1/16 = 0,0625 is our calculation factor.

PX            EM
  1          = 0,0625
 12 * 0,0625 = 0,75  
 48 * 0,0625 = 3 
250 * 0,0625 = 15,625

PX EM

1 = 0,0625

12 * 0,0625 = 0,75

48 * 0,0625 = 3

250 * 0,0625 = 15,625

The difference between EM and REM is the inheritance.

The REM value is calculated in reference to the root element, the font size of the HTML, not the BODY, element.

The EM value is calculated in reference to its parent element.

Due to limited support in older browsers, you can use graceful degradation in your stylesheet to use both.

li a {
font-size:20px;
font-size:1.25rem;
}

li a {

font-size:20px;

font-size:1.25rem;

}

The Pixel value must be added before the REM value !