A browsing context doesn't start its life empty. Instead, when a browsing context is created, if a JS program looks at what's in there, there's an about:blank doc in there. Since you can create a browsing context synchronously (e.g. document.body.appendChild(document.createElement("iframe"))), there has to be a way for the initial about:blank document to materialize synchronously. The HTML parser is always async. (Edit: The HTML parser is always async when loading a URL. Then there are innerHTML, createContextualFragment and DOMParser, which are synchronous.)

Add various events (readystatechange, DOMContentLoaded, load) to added fun. And the fact that browsing contexts that are top-level from the Web perspective are iframe-like from the XUL perspective and the code for dealing with this duality is a mess.