ALERT('OK')
<div id="HradyCzMap" style="width: 100%; height: 100%"></div>

<script>
/**
* Script pro distribuci map
* www.hrady.cz
* (c) 2009 Vlky
*/
//vlozeni divu pro mapu do stranky<br>
//otestovat
var beginzoom = 11; //nastaveno systemem bez moznosti ovlivneni

// ninimalni rozmery okna pro mapu
var minMapW = 470;
var minMapH = 250;

//div pro mapu
mapHrady = document.getElementById("HradyCzMap");
url = 'https://www.hrady.cz/mapy/wnd_mapa.php?beginx=49.155804&beginy=13.626008&beginzoom='+beginzoom+'&centerIcon=target&width='+mapHrady.offsetWidth;

console.log(url);

if (!mapHrady){
    alert ("Error!\nNo map DIV element defined!\nInsert <div id='HradyCzMap'></div> before script.");
}

else{
if (mapHrady.offsetWidth < minMapW || mapHrady.offsetHeight < minMapH ){
       alert ("Minimum size for DIV element is "+ minMapW + " x "+ minMapH+" pix!");
}else{
    mapHrady.innerHTML = '<iframe id="distIframeMap" name="distIframeMap" width="100%" height="100%" frameborder="0" scrolling="no" style="border:0px; width:100%; height:100%;" src="'+url+'"></iframe><div class="MapHradyInfoBox"></div>';
 }
}


</script>