Contacaratteri colorato

Devo fare in modo che quando arriva a dieci caratteri rimanenti, si colora di rosso

Codice:

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<script type="text/javascript"> 
charCount = 1; maxCharCount = 2000; 
function displayRemLength(fieldName) { 
remField = document.getElementById(fieldName); 
remField.innerHTML = (maxCharCount - charCount > 0) ? 
maxCharCount - charCount : 0; 
} 
function evalEntryLength(curField, maxLimit, discardXtra, errClass, normalClass {
maxCharCount = maxLimit; 
var fieldLength = getCharCount(curField); 
if (fieldLength > maxLimit) { if (errClass != "") { 
curField.className = errClass; 
}
if (discardXtra) { 
showAllowedLength(curField, maxLimit); 
} 
} 
else if (normalClass != "") { 
curField.className = normalClass; 
} 
} 
function getCharCount(curField) { 
charCount = curField.value.length; 
return charCount; 
} 
function showAllowedLength(curField, maxLimit) { 
curField.value = curField.value.substr(0, maxLimit); 
window.status = curField.value; 
} 
</script>

<span id="charCount">100</span> caratteri rimanenti

<textarea name="messaggio" id="text-guest" cols="0" rows="0" onkeyup="evalEntryLength(this, 100, true, '', ''); displayRemLength('charCount');></textarea>

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