ion of all HTML, SVG, or CSS tests both on the content side as well as the implementation side <em>as well as</em> testing the unique, EPUB 3.3 specific features. The standard checking tools that are widely used by the publishing community, like <a href="https://www.w3.org/publishing/epubcheck/">EPUBCheck</a> or <a href="https://daisy.github.io/ace/">ACE</a>, indeed include externally developed HTML, CSS, or accessibility checkers, which would check the validity or conformance of the publication resources.