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问题:

What exactly does "contentDocument" represent for an iframe (or even the old "frame" element)? Is it equivalent to the "html" element or the "body" element ? What is it's use? And is this property supported across all the browsers?

回答1:

w3.org

contentDocument of type Document, readonly, introduced in DOM Level 2 The document this frame contains, if there is any and it is available, or null otherwise.

MDN

From the DOM iframe element, scripts can get access to the window object of the included HTML page via the contentWindow property. The contentDocument property refers to the document element inside the iframe (this is equivalent to contentWindow.document), but is not supported by Internet Explorer versions before IE8.

msdn

the document this page or frame contains
This property is new in Windows Internet Explorer 8

So to get the innerHTML of the body element you could use

iframe.contentDocument.getElementsByTagName("body")[0]

iframe.contentDocument.body

in todays browsers.

回答2:

contentDocument is the standardized way to get hold of the iframe or frame's Document object. It is the same object as JavaScript running within the iframe would access via document.

As noted in other answers, IE didn't support it until version 8 but did support access to the iframe's Window object via contentWindow. A cross-browser way of getting hold of an iframe's <body> element is therefore:

var iframeDoc = iframe.contentDocument || iframe.contentWindow.document;
var iframeBody = iframeDoc.body;

Note that if the iframe is not served from the same domain as the main document, browser security restrictions will prevent access to its document object in this or any other way.

回答3:

contentDocument represents the document of an iframe (DOM object). It is not equivalent to html since documents have their own properties, however if you type: