Код получился вот такой:
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 {
width: 200px;
margin: 0;
padding: 0;
background-color: #2980b9;
}
.main-menu .sub-menu {
position: absolute;
width: 200px;
left: 180px; top: 0;
margin: 0;
padding:0;
list-style: none;
background-color: #1abc9c;
}
.main-menu .open {
position: relative;
background-color: #1abc9c;
}
.main-menu a {
display: block;
padding: 10px 15px;
text-decoration: none;
color: #ffffff;
border-bottom: 2px solid #2c3e50;
}
.sub-menu a {
border-bottom-color: #2ecc71;
}