只有a标签

a {
    border-radius: 10px;
    padding: 5px;
    width: 12rem;
    height: 42px;
    line-height: 35px;
    margin: .8rem;
    font-size: 12px;
    display: inline-block;
    text-decoration: none;
    text-align: center;
    background: rgba(0,0,0,.35);
    color: #fff;
    font-weight: 700;
}
   a:hover {
        color: deepskyblue;
        background-color: black;
    }

a标签+li标签

a {
    font-weight: 700;
    display: block;
    background: rgba(0, 0, 0, 0.35);
    color: rgb(255, 255, 255);
    font-size: 12px;
    text-align: center;
    line-height: 35px;
    padding: 4px 0px;
    border-radius: 2px;
    transition: all 0.2s ease 0s;
    z-index: 1;
      text-decoration: none;
}

a:hover {
    color: deepskyblue;
    background-color: black;
}


 li {
    position: relative;
    height: 42px;
    width: 110px;
    margin: 3px;
    list-style: none;
    float: left;

}

nav+a+ul+li标签，加框线

a {border-left: solid 5px #f00;
    display: block;
    min-width: 7.5rem;
    height: 2.75rem;
    line-height: 2.75rem;
    padding: 0 1.25rem 0 1.45rem;
    letter-spacing: 0.2rem;
    font-size: 0.8rem;
  font-weight: 700;
    color: #fff;
    text-decoration: none;
}
a:hover {
    color: deepskyblue;
    background-color: black;
}
nav  {
    display: flex;
    -webkit-flex-direction: column;
    align-items: center;
    text-align: center;
}
   



ul {  
      margin: 0px;
    padding: 0px;
    font-size: 14px;
    font-weight: 700;
    background: rgba(0,0,0,.45);
    display: flex;
    list-style: none;
    border: solid 5px #f00;
}