#mainmenu{
    width: 150px;
    padding:0;
    margin: 0;
    list-style: none;
}
#mainmenu ul>li>ul{
    display: none;
    left: 100%;
    top:0;
    position: absolute;
    width: max-content;
    padding: 0;
    margin: 0;
}
#mainmenu li{
    list-style: none;
    background-color:#000000;
    padding: 0;
    margin: 0;
    width: 100%;
    border-bottom: 1px solid #FFF;
    position: relative;
}

#mainmenu a{
    display: block;
    color: white;
    box-sizing: border-box;
    width: 100%;
    padding: 10px 20px;
    text-decoration: none;
    
}

#mainmenu a:hover{
    background-color: darkgray;
}
#mainmenu li:hover>ul{
    display: block;
}