1: https://developer.mozilla.org/en/docs/Web/CSS/padding MDN
Also see this question, "Why does CSS not support negative padding?" and his answers.
Also please consider The Box Model when using padding. Depending on the box-sizing value, padding on an element can either add to the previously defined height/width of an element or not.
Related Properties:
Padding on inline elements will only apply to the left and right of the element, and not the top and bottom, due to the inherent display properties of inline elements.