Reakce na odpověď

Áno, JavaScript vkladám pomocou tlačidla pre JavaScript, ktoré obalí kód do [code lang="js"][/code] (formátovanie sa dá vyskúšať pri náhľade).

Tvoj kód funguje aj v IE.

Doplním ešte do minulého kódu JavaScript:

var helper =
{
	addClass: function (el, className)
	{
		this.removeClass (el, className);
		
		el.className += " " + className;
	},
	
	removeClass: function (el, className)
	{
		var cls = el.className.split (/\s+/);
		var ar = [];
		
		for (var i = cls.length; i > 0;)
			if (cls [--i] != className)
				ar [ar.length] = cls [i];
		
		el.className = ar.join (" ");
	},
	
	hasClass: function (el, className)
	{
		var cls = el.className.split (/\s+/);
		
		for (var i = cls.length; i > 0;)
			if (cls [--i] == className)
				return true;
		
		return false;
	}
};

CSS:

.invalid { background: red; }

HTML:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
	<head>
		<meta http-equiv="content-type" content="text/html; charset=utf-8" />
		<title>Test</title>
		<link href="subor-so-stylmi.css" type="text/css" rel="stylesheet" />
		<script type="text/javascript" src="subor-s-javascriptom.js></script>
	<body>
		<form id="editor" action="">
			<p>
				<input type="text" name="a" value="13" class="validate" />
			</p>
		</form>
	</body>
</html>

Změnit barvu textu za chodu.