what's the difference bewteen document.defaultView.getComputedStyle and window.getComputedStyle
<p>when get a element's style, we always use </p>

<p><code>if(document.defaultView &amp;&amp; document.defaultView.getComputedStyle)</code> to check whether the browser support the method or not.</p>

<p>why not use <code>if(window.getComputedStyle)</code>?</p>
