position: absolute without setting top/left/bottom/right?

The standard generally says if top/bottom, left/right are auto, then default them to their position: static values:

http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width

http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-height

AFAIK, you should pay attention of hierarchical css rules, beacuse if you don't specify top, left and other attributes, they are inherited from parent element, or are set by defaults in browser's css. IMHO, it's better to initialize elements with your values.