style.css

*{
    margin: 0;
    padding: 0;
}
.nav{
    background-color: blue;
    width: 100%;
    height: 50px;
    margin: 100px auto;
}
.topnav{
    height: 50px;
    width:auto;
    margin-left: 150px;
    list-style:none;
}
.topnav__list{
    width: 100px;
    height: 50px;
    float: left;
    background-color: blue;
    font-size: 14px;
    text-align: center;
    line-height: 50px;
}
.topnav__list:hover{
color: #fff;
background-color: skyblue;
}
.topnav__link{
    color: #fff;
    text-decoration: none;
    font-weight: bold;
}
.subnav {
    height: 50px;
    width:auto;
    list-style:none;
    display: block;
    position: absolute; 
    width: auto; 
}
.subnav__list {
    float: left; 
    display: none;
    height: 50px;
    width:100px;
    background-color: skyblue;
    padding:5px 5px;
    font-size: 14px;
    line-height: 50px;
}
.subnav__link {
    display: flex;
    flex-direction: row;
    color: #fff;
    text-decoration: none;
}
.topnav__list:hover .subnav__list{
    display: block;
}
.subnav__link:hover {
    text-decoration: underline;
}