HTML5 placeholder and required for IE9

validate.js

 //from http://kamikazemusic.com/quick-tips/jquery-html5-placeholder-fix/#comment-938
$(document).ready(function() {
	if (Modernizr.input.placeholder)
		return;

	$('input[placeholder]').focus(function() {
		if ($(this).hasClass('placeholder')) {
			if ($(this).val() == $(this).attr('placeholder'))
				$(this).val('');
			$(this).removeClass('placeholder');
		}
	});

	$('input[placeholder]').keypress(function() {
		if ($(this).hasClass('placeholder')) {
			if ($(this).val() == $(this).attr('placeholder'))
				$(this).val('');
			$(this).removeClass('placeholder');
		}
	});

	$('input[placeholder]').blur(function() {
		if ($(this).val() != '')
			return;
		$(this).addClass('placeholder');
		$(this).val($(this).attr('placeholder'));
	});

	$('input[placeholder]').each(function() {
		if ($(this).val() != '' && $(this).val() != $(this).attr('placeholder'))
			return;
		$(this).val($(this).attr('placeholder')).addClass('placeholder');
	});

	$('form').submit(function() {
		$(this).find(".placeholder").each(function() {
			$(this).removeClass('placeholder');
			$(this).val('');
		});
		validateform($(this));
	});
});
function validateform(form){
	if (Modernizr.input.required)
		return true; //already handled
		
	var valid = true;
	form.find("input[required]").each(function(){
		if($(this).val().length == 0) {
			valid = false;
			$(this).addClass('redborder');
		} else {
			$(this).removeClass('redborder');
		}
	});
	return valid;
}

adapted from http://kamikazemusic.com/quick-tips/jquery-html5-placeholder-fix/#comment-938

todo: add in email validation as well for input type="email"

I added it to the bottom of plugins.js (if you're using html5 boilerplate), otherwise at the bottom of your javascript file. Don't add it to your existing $(document).ready() though. Requires jQuery and Modernizr.