Google's crawlers don't really interpret JS, so they won't see all the extra span tags that this script applies. In other words, Google should interpret the header on the demo page as:

    <header>  
        <h1>Kerning.js</h1>  
        <h2>Take control of your web typography.</h2>  
    </header>  

Check out http://stackoverflow.com/questions/2061844/does-googles-craw... for the usual caveats.