CSS float
[추천] CSS Float속성의 모든 것!
http://hanulnun.egloos.com/2863671
CSS Float속성의 모든것 (All About Floats)
http://techbug.tistory.com/181
How To Clear Floats Without Structural Markup
http://www.positioniseverything.net/easyclearing.html
float을 clear하는 4가지 방법.
http://naradesign.net/wp/2008/05/27/144/