From http://developer.yahoo.com/yui/grids/:
On the advice of our designers, the 100% grid (#doc3) has 10px of margin on the left and right sides. This prevents the content from bleeding into the browser's chrome. If you prefer, you can set it back to zero by adding this to your document:
#doc3 {margin:auto;}
a lot of ppl also complain that yui is as verbose as tables... not to mention the M$ buyout is looming over my head. I'm wondering if this will affect all of Yahoo's open source efforts, since M$ still isn't very friendly towards open source (yui becomes ie centric)