Måske sådan:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"
http://www.w3.org/TR/html4/loose.dtd"><html>
<head>
<title>Vis billedtekst</title>
<meta name="keywords" content="søgeord adskilt af komma">
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<style type="text/css">
</style>
<script language="javascript" type="text/javascript">
function checkTxt(fld){
var txt = fld.value,l=txt.length,d = document.getElementById('minDiv'),works=false;
for(i=0;30-l>=i;i++)works=works||/^[\- ]$/.test(txt.charAt(15-i));
if(30>=l&&works){
d.innerHTML = txt;
d.style.color = 'lightgreen';
}else{
d.innerHTML = "Teksten er ugyldig som billedtekst ...";
d.style.color = 'red';
fld.value = fld.defaultValue;
}
}
</script>
</head>
<body>
<form id="minForm" name="minForm">
Skriv din billed-tekst (Max. 30 tegn i to linjer !-)<br>
<textarea id="mitTextarea" name="mitTextarea" onchange="checkTxt(this);" onfocus="if(this.value==this.defaultValue)this.value='';" onblur="if(this.value=='')this.value=this.defaultValue;" cols="15" rows="2" style="overflow:hidden;">Skriv billed-tekst her</textarea>
</form>
<div id="minDiv" style="width:135px;height:42px;font-family:courier">Eksempel-tekst</div>
</body>
</html>