</a></h2>
   <p>The <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="custom-state-pseudo-class">custom state pseudo class</dfn> <span class="css">:--foo</span> is a <a data-link-type="dfn" href="https://drafts.csswg.org/selectors-4/#pseudo-class" id="ref-for-pseudo-class②">pseudo-class</a>,
and applies while an element has a certain state. "State" is a per-element
information which can change over time depending on user interaction and other
extrinsic factors. The <a data-link-type="dfn" hre