float对块有什么影响
两个div是一样的，div2加上float:right;后下面多出20px,怎么回事呢？
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Simple Fixed Width Rounded Corner Box</title><style type="text/css"><!--/* pretty stuff================================== */body { font: 62.5%/1.6 "Gill Sans", Futura, "Lucida Grande", Geneva, sans-serif; color: #666; background: #fff; }h2 { font-size: 2.2em; font-weight: normal; line-height: 1; color: #94b767; margin: 0;}.box { font-size: 2em;}.box2 { font-size: 2em;}/* rounded corner box================================== */.box { width: 418px; background: #effce7 url(images/bottom.gif) no-repeat left bottom;}.box h2 { background: url(images/top.gif) no-repeat left top;}.box h2 { padding: 10px 20px 0 20px;}.box p { padding: 0 20px 10px 20px;}/*================================== */.box2 { float:right; /* 脱离文档流，究竟发生了什么事 */ /* position:absolute; left:500px; */ width: 418px; background: #effce7 url(images/bottom.gif) no-repeat left bottom;}.box2 h2 { background: url(images/top.gif) no-repeat left top;}.box2 h2 { padding: 10px 20px 0 20px;}.box2 p { padding: 0 20px 10px 20px;}--></style></head><body><div class="box2"> <h2>Lorem Ipsum2</h2> <p>Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Proin venenatis turpis ut quam. In dolor. Nam ultrices nisl sollicitudin sapien. Ut lacinia aliquet ante.</p></div><div class="box"> <h2>Lorem Ipsum</h2> <p>Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Proin venenatis turpis ut quam. In dolor. Nam ultrices nisl sollicitudin sapien. Ut lacinia aliquet ante.</p></div></body></html>