JSFiddle - React, Tailwind, and code Playground

HTML

<div id="header"> <a class="nav-item"><span>START</span></a>
 <a class="nav-item"><span>GRUPPEN</span></a>
 <a class="nav-item"><span>HILFE</span></a>

</div>

CSS

#header {
    margin-top:2em;
}
.nav-item {
    width: 120px;
    display:inline-block;
    position:relative;
    font-family:arial, helvetica;
    text-align:center;
    height:3em;
    line-height:3em;
    margin:0 1.35em;
    cursor:pointer;
}
.nav-item span {
    position:relative;
    z-index:2;
}
.nav-item:before {
    content:"";
    width:100%;
    height:3em;
    padding:0 1.5em;
    position:absolute;
    top:0;
    left:-1.5em;
    transform: skew(45deg);
}
.nav-item:hover {
    background:black;
    color:white;
}
.nav-item:hover:before {
    background:black;
}