b div在a div中，在b中使用margin-top 为什么a也受到影响？
<html><head> <title>TestDiv</title> <style type="text/css"> *{ padding:0; margin:0; } #a{ width:960px; height:700px; background-color:red; margin: 0 auto; } #b{ width:960px; height:50px; background-color:green; margin-top: 10px; } </style></head><body> <div id="a"> <div id="b"> </div> </div></body></html><html><head> <title>TestDiv</title> <style type="text/css"> *{ padding:0; margin:0; } #a{ width:960px; height:700px; background-color:red; margin: 0 auto; } #b{ width:960px; height:50px; background-color:green; margin-top: 10px; float:left; display:inline; } </style></head><body> <div id="a"> <div id="b"> </div> </div></body></html>