Whenever IE6 encountered a problem with a HTTPS-delivered webpage, the user was informed via a modal dialog box and was asked to make a security decision.  IE7 follows the XPSP2 “secure by default” paradigm by defaulting to the secure behavior.
Most importantly, IE7 will block navigation to HTTPS sites that present a digital certificate that has any of the following problems:
   1. Certificate was issued to a hostname other than the current URL’s hostname
   2. Certificate was issued by an untrusted root
   3. Certificate is expired
   4. Certificate is revoked
Upon encountering a certificate problem, IE7 presents an error page that explains the problem with the digital certificate.  The user may choose to ignore the warning and proceed in spite of the certificate error (unless the certificate was revoked).  If the user clicks through a certificate error page, the address bar will floodfill with red to serve as a persistent notification of the problem.