There is no standard about that, but port 8443 is sometimes known as the http-alt, and it seems the only one being popularly used as alternative HTTPS port.
It may have become popular because of mod_nss (an HTTPD module alternative to mod_ssl) using it on its default configuration.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…