HTMLIFrameElement.contentDocument

If the iframe and the iframe's parent document are Same Origin, returns a Document (that is, the active document in the inline frame's nested browsing context), else returns null.

Example of contentDocument

const iframeDocument = document.querySelector("iframe").contentDocument;

iframeDocument.body.style.backgroundColor = "blue";
// This would turn the iframe blue.

Specifications

Specification
HTML Standard # dom-iframe-contentdocument

Browser compatibility

BCD tables only load in the browser