                 <p>
                      Structural constraints on a the EPUB-specific files of the publication; examples include the <a href="https://www.w3.org/TR/epub-33/#sec-ocf">structure of the container format (OCF)</a> or of the <a href="https://www.w3.org/TR/epub-33/#sec-package-doc">package document</a>.
                    </p>
                </li>
                <li>
                    <p>
                      Extra restrictions concerning content documents such as the <a href="https://www