<form name="formular" id="formular" action="#">
<div id="mestoDiv">Město: <input id="mesto" name="mesto"></div>
<div><input id="odesliFormular" type="submit"></div>
</form>

function zkontrolujFormular() {
    var mesto = document.forms[0]["mesto"];
    if (mesto.value != "Praha") {
        var mestoDiv = document.getElementById("mestoDiv");
        mestoDiv.style.fontWeight = "bold";
        return false;
    } else {
        return true;
    }
}

<script type="text/javascript" src="formular.js"></script>

<script type="text/javascript">
    function inicializace() {
        document.forms[0].onsubmit = function() { return zkontrolujFormular(this) };
    }
    window.onload = inicializace;
</script>

