It’s impossible to change the validation style with only HTML5/CSS3.
It’s part of the browser. The only attribute I figured out to change is the error message by using this example:
document.getElementById("name").setCustomValidity("Lorum Ipsum");
But, as shown in this example : http://jsfiddle.net/trixta/qTV3g/, you can override the panel style by using jQuery. This is not a plugin, it’s a core functionality, uses a DOM lib called Webshims and, of course, some CSS to style the popups.
I found that very useful example in this bug post titled Improve form validation error panel UI
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I think this is the best solution you can find and only way to override the basic (ugly) error panel.
Regards.