example.css

body {
    box-sizing: border-box;
    max-width: 90vw;
    margin: auto;
    padding: 4vh 0;
}

.selext {
    float: left;
    width: 100%;
    position: relative;
}

.selext--label {
    border: 1px solid #333;
    padding: 10px 25px;
    display: inline-block;
    width: 300px;
    box-sizing: border-box;
    cursor:pointer;
}

.selext--list {
    margin: 0;
    padding: 0;
    list-style: none;
    display: block;
    width: 300px;
    border-left: 1px solid #333;
    border-bottom: 1px solid #333;
    border-right: 1px solid #333;
    box-sizing: border-box;
    padding: 0 15px;
    overflow:hidden;
}

.selext--item {
    float: left;
    width: 100%;
    padding: 5px 10px;
    transition:all 0.35s ease-out;
    background-color:white;
    box-sizing: border-box;
    cursor: pointer;
}

.selext--item[selected]{
  font-weight:bold;
}

.selext--item:hover{
  background-color: #f5f3f3;
  transition:all 0.15s ease;
}

.selext--button {
    position: absolute;
    top: 0;
    right: 0;
}

.selext[dropdown="closed"] .selext--list{
  height:0;
}

.selext[dropdown="opened"] .selext--list{
  height:auto;
}