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• JavaScript
• HTML
• CSS
• Result

window.addEvent('domready', function(){

  $('select_me').addClass('selected');

});

<h1 id="select_me">Select me and add the `selected` class</h1>
<h1>Leave me alone</h1>

h1.selected {
    color: #1b609a;
}

/* irrelevant */
body {
    font-family: helvetica;
}