Why does the following div cover the previous div? If float:left is also added to the back, it will not be covered_html/css_WEB-ITnose

float:left; HTML css div

#divmain{
margin:0 auto;
width:470px;
}
    #div1{
float:left;
width:100px;
height:100px;
background:#006;
}
 #div2{

width:100px;
height:100px;
background:#CCC;

}
        






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