This is a perfect summary of why I find CSS to be so irritating 
"For instance, width has absolutely no effect on inlines. width:auto shrinkwraps floats to the width of their content. width:auto shrinkwraps absolutes when left and right are set to auto. width:auto stretches blocks to the width of their parent element. width:auto stretches absolutes to the width of their containing block when left and right are set to 0. width:100% stretches blocks and floats to the width of their parent element as long as they do not have borders, padding, and margins. width:100% stretches tables to the width of their parent even if they do have borders and padding. width:100% stretches absolutes to the width of their closest positioned ancestor instead of their parent. width:100em sizes an element in relation to the height of its font-size, which allows the element to be sized wide enough to contain a certain number of characters. width:100px sizes an element to a fixed number of pixels regardless of the font-size of its text." -- Michael Bowers, Pro HTML5 and CSS3 Design Patterns
