body {
}
.zahlavi {
    width: 100%;
    background-color: aqua;
    height: 100px;
    text-align: center;
    font-size: 40px;
}
.clanek {
    box-sizing: border-box;
    width: 100%;
    height: 600px;
    margin: 20px 0;
    padding: 15px;
    background-color:antiquewhite;
}
@media screen and (min-width: 900px) {
    .clanek {
        width: 79%;
        float: left;
    }
}
.postrannipanel {
    box-sizing: border-box;
    width: 100%;
    height: 200px;
    margin: 20px 0;
    padding: 10px;
    background-color: aliceblue;
}
@media screen and (mid-width: 900px) {
    .postrannipanel {
        width: 19%;
        float: right;
    }
}
.panel_1 {
    text-align: center;
    background-color: aquamarine;
    height: 229px;
}
.panel_2 {
    text-align: center;
    background-color: aquamarine;
    height: 230px;
}