Originally Posted by DaleDe
Actually there is a guide like that. Authors should not define those things that the user should have control over. This is spelled out very well for Mobipocket format and could easily be adopted for ePUB.
But then one thing is what the user "should have control over", and another thing is what the user "has control over". Take fonts or justification. In ADE there's no way to change them, so if the defaults are not enough for your book (for instance, if you have Cyrillic characters), you had to define it (embed a font with Cyrillic characters). Now the Cybook firmware allows the user to change fonts and justification, so this is not needed, but other readers still don't support this.