body {
}
.zahlavi{width:100%; background-color:azure;height:50px;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 (mind-width: 900px) {.clanek{width:79%;float:left}}

.postranni-panel{
    box-sizing:border-box;
    width:100%;
    height:200px;
    margin: 20px 0;
    padding:10px;
    background-color: #F7B492;
}
@media screen and (min-width:900px) {
    .postranni-panel{
        width:19%;
        float: right;
    }
}
.panel_1{text-align:center;background-color:blanchedalmond}
.panel_2{
    text-align:center;
    background-color:lightblue
}