Regarding CSS

Pixels are browser dependent. It is the absolute size that you would see on your screen.

Em are sort of like percentages.Ems is referring to the base text size. The value of 1 em means the same thing as a value of 100 percent. But you can also say it in the opposite way: A percentage value is just an em multiplied by 100.

Points(pt) are what you would want to use in print media.


in RWD mainly we uses % instead of absolute size and media queries also...
for more info about RWD see below thread..
http://learn.shayhowe.com/advanced-html-css/responsive-web-design/


Please mark it as answer if it helps...