DIV+CSS the outer background of the color disappeared solution

<div class= "header" ><div class= "Header_left" >header_left</div><div class= "Header_right" > header_right</div></div>.header{width:1000px;background:red;}. header. Header_left{width:600px;background:green;float:left;}. header. Header_right{width:400px;background:pink;float:right;}. Clear{clear:both, this div+css is  normal in IE and chrome, but the background color of the header in Firefox is not shown, add <div class= after the. Header_right tag. Clear "></DIV> the background color of the outer layer is not a problem, but there is a blank line above the. Header, and the solution found online is to add Overflow:hidden to the CSS style of the outer div; The result can really solve the problem, but the cause  O (∩_∩) o~  

DIV+CSS the outer background of the color disappeared solution