IEXPLORE.EXE is only a small exe file. The rest of Internet Explorer is implemented in system libraries that we need to write. Many applications use these libraries to show HTML. The most important of these libraries are:
Implements WebBrowser control that is used by applications to embed OLE documents (usually HTMLDocument). See ShDocVw.
Implements HtmlHelp. It should be implemented on top of the WebBrowser control. Currently it's just a stub.
Implements URLMoniker and a relative API that is used to abstract downloading process. Currently there are mostly hacks or stubs. See UrlMon.
Basic functionality of MSHTML is in GIT. See the MozillaIntegrationSummerOfCode2005Summary.