.abas {
position: relative;
}
.aba {
display: inline;
}
.aba > a {
float: left;
padding: 0.5em 1em;
background: linear-gradient(#FFF, #EEE);
border-width: 1px;
border-style: solid;
border-color: #CCC #CCC #FFF;
border-top-right-radius: 8px;
border-top-left-radius: 8px;
}
.aba:not(:target) a {
border-bottom: 0 none;

}
.aba:target a, a:hover {
background: white;
}
.conteudo {
position: absolute;
left: 0;
top: calc(2em + 4px); /* altura do link + bordas */
z-index: -2;
border: 1px solid #CCC;
background-color: white;
}
.aba:target .conteudo {
z-index: -1;
}