28/28 100%, проверьте, пожалуйста!!!

html,
body {
margin: 0;
padding: 0;
}

body {
width: 380px;
min-height: 280px;
padding: 10px;

font-size: 14px;
line-height: 1.4;
font-family: “Arial”, sans-serif;

background-color: #ecf0f1;
}

.main-menu{
margin:0;
padding:0;
width:200px;
list-style:none;
background-color:#2980b9;
}

.main-menu a{
color:#ffffff;
display:block;
padding:10px 15px;
text-decoration:none;
}

.main-menu > li {
position: relative;
border-bottom:2px solid #2c3e50;
}

.main-menu > .open > a{
background-color:#1abc9c;
}

.main-menu .open .sub-menu{
display:block;
}

.main-menu .sub-menu{
display:none;
margin:0;
padding:0;
width:200px;
top:0;
left:180px;
z-index:10;
list-style:none;
position:absolute;
background-color:#1abc9c;
}

.main-menu .sub-menu a{
border-bottom:2px solid #2ecc71;
}

.main-menu > li:hover .sub-menu{
display:block;
}