The HTML Email part of the Zend Mail documentation currently 404s: http://framework.zend.com/manual/1.12/en/zend.mail.html-mails.html
This page is linked to from the main Zend Mail table of contents (http://framework.zend.com/manual/1.12/en/zend.mail.html). I've selected 1.12 as the affected version but I think this is a general problem.
Comments on the Zend_Mail docs page suggest this has been an issue for a while.
This issue was ported from the ZF2 Jira Issue Tracker at
Known GitHub users mentioned in the original message or comment:
This page is no longer 404'ing so this issue must have been resolved by another fix to the documentation.
Problem has been fixed.