.zahlavi {
    width: 100%;
    background-color: aqua;
    height: 60px;
    text-align: center;
    font-size: 40px;
    margin-left: auto;
    margin-right: auto;
}
.clanek{
    box-sizing:border-box;
    width:100%;
    height:600px;
    margin:20px 0;
    padding:15px;
    background-color:aqua;
    }
@media screen and (min-width: 900px) {
    .clanek {
        width: 79%;
        float: left;
    }
}
.posranipanel {
    box-sizing: border-box;
    width: 100%;
    height: 150px;
    margin: 20px 0;
    padding: 10px;
    background-color: aqua;
}

    @media screen and (min-width: 900px) {
        .posranipanel {
            width: 19%;
            float: right;
        }
    }