JSValidate

var js_options = {
	errorTag: "span", // which tag do you want to use for error container. it must be one that opens and closes (div,span,p,b)
	errorClass: "jsvalidation", // this is the css class name given to the tag above
	errorLocation: "afterEnd", // only accepts beforeBegin or afterEnd (either before or after the input element)
	//note: if you choose "none" for the above attribute, you must create the error yourself and give the element an ID consisting of the option below + the name of the field to validate.
	errorIDPrefix: "jsvalidator", // prefix of the id of the element above that will attach to the name or id of the form element. don't use spaces or special characters.
	startGone: false, //couldn't think of another name for this, but if true, it will apply "display:none", otherwise, the element is just invisible.
	useBR: "none", // accepts before, after, both or none; This will add a new line (<br />) before and/or after the above element.
	useBlur: true, // this will attach an onBlur validator to each form element.
	submitClass: 'submit_action', // apply this class inside any form to let this element submit the form.
	highlightColor: '#FFFF99', //default color should be:  #FFFF99
	endColor: '#FFFFFF', //this is what you generally want to set to the background color behind the form elements.
	extMessage: true // if true, and you have accept value on file input, it tells user what extensions are accepted.
}

<script type="text/javascript" language="javascript" src="scriptaculous/lib/prototype.js"></script> <script type="text/javascript" language="javascript" src="scriptaculous/src/scriptaculous.js"></script> <script type="text/javascript" language="javascript" src="jsvalidate.js"></script>
<form name="query" method="get" action="/demo/">
	Search: <input type="text" name="search" class="jsrequired" /><br />
          <input type="submit" value="go" />
</form>