The user wants the website to work in IE6, developing and testing only against IE6 to the detriment of other browsers is not generally regarded as a healthy state of affairs.

The standard exists, it is the responsibility of both the producer and consumer of ePUB files to adhere to the standard.

https://en.wikipedia.org/wiki/Robustness_principle

If a large fraction of your users are on IE6 and you can't realistically get them off it, you need to make sure your site works in IE6, and good tooling should help you do this. Of course you also want to make sure it works in other browsers your users use, and a standard may be helpful in doing that.