I have a website in HTML5 that needs to validate. It uses an iframe, which is valid, but the frameborder attribute is not. And the CSS border property doesn't work. See for yourself:

HTML Code:
     <meta charset="utf-8" />
<style type="text/css">
#contentContainer {
    height: 200px;
    width: 200px;
#I-frame {
    height: 100%;
    width: 100%;
    border: 0; /* doesn't work */
    <div id="contentContainer">
        <iframe name="I-frame" id="I-frame" src="introduction.html">
Can I insert the attribute frameborder="0" with javascript? If so, how? I've been fiddling with this method for a couple of hours, but have not succeeded so far. The full(y adjusted) HTML + JS code would be appreciated, to save discussion time.

Thanks in advance.