LayoutTests/fast/frames/css-style-declaration-isolation.html

<iframe id="ifr"></iframe>
  This tests that modifications to the prototype of CSSStyleDeclaration don't
  modify its prototype in another document.
<div id="result"></div>

<script>
if (window.testRunner)
    testRunner.dumpAsText();

CSSStyleDeclaration.prototype.foo = "foo";
var win = document.getElementById('ifr').contentWindow;
var result = win.CSSStyleDeclaration.prototype.foo;
if (result)
    document.getElementById('result').innerText = "FAIL: expected 'undefined', got '" + result + "'";
else
    document.getElementById('result').innerText = 'PASS';
</script>