<textarea id="text" name="text" maxlength="1250"></textarea>

 This  is the complete script. Call setMaxLength() onload, and enjoy the effect.

function setMaxLength() {
	var x = document.getElementsByTagName('textarea');
	var counter = document.createElement('div');
	counter.className = 'counter';
	 for  (var i= 0 ;i<x.length;i++) {
		 if  (x[i].getAttribute('maxlength')) {
			var counterClone = counter.cloneNode(true);
			counterClone.relatedElement = x[i];
			counterClone.innerHTML = '<span>0</span>/'+x[i].getAttribute('maxlength');
			x[i].parentNode.insertBefore(counterClone,x[i].nextSibling);
			x[i].relatedElement = counterClone.getElementsByTagName('span')[ 0 ];

			x[i].onkeyup = x[i].onchange = checkMaxLength;
			x[i].onkeyup();
		}
	}
}

function checkMaxLength() {
	var maxLength =  this .getAttribute('maxlength');
	var currentLength =  this .value.length;
	 if  (currentLength > maxLength)
		 this .relatedElement.className = 'toomuch';
	 else 
		 this .relatedElement.className = '';
	 this .relatedElement.firstChild.nodeValue = currentLength;
	// not innerHTML
}