Everything about float CSS property.
http://css.maxdesign.com.au/floatutorial/tutorial0103.htm
http://www.smashingmagazine.com/2007/05/01/css-float-theory-things-you-should-know/

Here are a few great concepts of creating boxes with rounded corners with CSS only:

Pure CSS:

• http://www.spiffycorners.com/
  
• http://www.cssplay.co.uk/boxes/snazzy.html
• http://www.fireandknowledge.org/archives/2004/05/22/
  rounded-corners-without-images-part-1/

Other Solutions:

• http://www.vertexwerks.com/tests/sidebox/
• http://www.editsite.net/blog/rounded_corners.html
• http://acko.net/blog/anti-aliased-nifty-corners
• http://seky.nahory.net/2005/04/rounded-corners/

http://www.w3.org/QA/2002/04/valid-dtd-list.html