I bet all the certificate metadata shown in the „View a certificate“ popup window is vulnerable to cross-site scripting. Back then you probably wouldn’t get a <script> tag through a CA's review process and I found such a problem in Netscape's image „About page“ popup.
If it were vulnerable to XSS, why would you even want it properly signed by a CA? People almost never inspect the certificates of working websites, the only time they might look at it is when it fails validation.