box-sizing: border-box

Really sucks to go into someone else’s css to add some stuff and halfway there realizing they are not using border-box. 

Go ahead and make your life easier by slapping this code at the top of your stylesheet:

/* apply a natural box layout model to all elements */
* { -moz-box-sizing: border-box; -webkit-box-sizing: border-box; box-sizing: border-box; }

Read this from Paul Irish.