<script>
function c(f){
if(!f.c1.checked){alert();return false}
return true;
}
</script>
<form onsubmit="return c(this)">
<input type=checkbox name=c1><input>
<input type=submit>
</form>
function c(f){
if(!f.c1.checked){alert();return false}
return true;
}
</script>
<form onsubmit="return c(this)">
<input type=checkbox name=c1><input>
<input type=submit>
</form>
function c(){
if(form1.chk.value="off"){alert();return false}
return true;
}
</script>
<form onsubmit="return c()">
<input type=radion name=c1 onclick="document.form1.chk.value="on">
<input type=hidden value="off" name="chk">
<input type=submit>
</form>