the problem in using of (mx) VideoDisplay.attachCamera(camera) in flex webapp

i have encountered a problem last night. my code looks like the following:

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
               xmlns:s="library://ns.adobe.com/flex/spark"
               xmlns:mx="library://ns.adobe.com/flex/mx"
               minWidth="955" minHeight="600">
    <fx:Declarations>
        <!-- Place non-visual elements (e.g., services, value objects) here -->
    </fx:Declarations>
    <fx:Script>
        <![CDATA[
            private function onCreateComplete():void
            {
                var camera:Camera = Camera.getCamera();

                if(camera)
                     vid.attachCamera(camera);
            }
        ]]>
    </fx:Script>
    <mx:VideoDisplay id="vid" width="300" height="300"
                     verticalCenter="0" horizontalCenter="0"
                     creationComplete="onCreateComplete()"/>
</s:Application>

when i run the code above in my firefox, the videodisplay component showed in the browser,

but the video captured by my camera didn't show in the videodisplay.

if i change the <s:application> to <s:windowedapplication>, and run as a desktop app, everything is OK.

then what is the problem of not showing the video captured by my camera in flex webapp?

BTW, my flashbuidler's flex SDK is 4.5.