Почему margin auto не работает

'

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'
'body{
margin: 0;
padding: 0;
background: #41c6b7;
}
*{
font-family: arial, sans-serif ;
color: #a1a1a1;
}

/*header*/
.header{
background: white;
}
.LOGO,.SEARCH,.CART{
height: 85px;width:455px;float: left;
}

.logo{margin: 20px 0 0 140px}
form{margin: 25px 0 0 0 }
.cart{margin: 20px 10px 0 150px }

.SEARCH input[type="search"]{
width:436px;height:32px;
border: 1px solid #e3e3e3;
outline: none;
}
.SEARCH input[type="submit"]{
position: relative;left:404px;top:-32px;
width: 32px;
height: 32px;
border:none;outline: none;
background: url(image/sear.png) no-repeat 50% 50%;
cursor: pointer;
}
.CART strong{font-size: 30px;}

/*navmenu*/
.TOPMENU{
width:100%;
}
.TOPMENU ul{
margin:0 auto;
}
.TOPMENU ul li{
float:left;list-style: none;
}
.TOPMENU ul li a{
display: block;
background-color: #4ccfc1;
color: #fff;
text-decoration: none;
padding: 10px;
}
.TOPMENU ul li a:hover{
background: #3cc395;
}
.cont{clear: both;}'