header{
    text-align: center;
    
}
.nav{
    
    display:flex;
    text-align:center;
    flex-direction:column;
    list-style-type:none;
}

.nav, header{
    
    order:-1;
    flex-direction:row;
    
}

ul li {
    
    display:inline; 
    flex:auto;
    text-align:center;
    background: #ccc;
    color:black;
    margin: 0 5px;
    padding: 5px 0;
    text-decoration:none;
    border: 1px solid black;
    
}

ul li:hover{
    outline: 1px solid blue;
    color: blue;
    text-decoration: underline;
    flex-grow:2;
    
}
.parent{
    
    display:flex;
    flex-wrap: wrap;
    border-bottom:0.5rem;
    border-left: solid black 3px;
    border-right: solid black 3px;
    text-align:center;
    
}

#a1{
    
    flex:33%;
    border-right: solid black 3px;
    border-bottom: solid black 3px;
    
}

#a2{
    
    flex:33%;
    border-right: solid black 3px;
    border-bottom: solid black 3px;
    
}

#a3{
    
    flex:33%;
    border-right: solid black 3px;
    border-bottom: solid black 3px;
    
}