 <p>This may be late, but I suggest trying <code>document.execCommand('fontsize,false,'5').</code> In some browsers, not passing <code>false</code> to defaultUI (the second argument) results in the command not running. </p> <p>What does it return when you run it? <code>execCommand</code> will return <code>true/false</code> based on success or failure.</p>