You should always set a width on floated items (except if applied directly to an image - which has implicit width). W3C's Cascading Style Sheets, level 2, CSS2 Specifications states:
A floated box must have an explicit width...
If no width is set, the results can be unpredictable. Theoretically, a floated element with an undefined width should shrink to the widest element within it. This could be a word, a sentence or even a single character - and results can vary from browser to browser.