s://webidl.spec.whatwg.org/#idl-undefined" id="ref-for-idl-undefined"><c- b>undefined</c-></a> <a class="idl-code" data-link-type="method" href="#dom-customstateset-add" id="ref-for-dom-customstateset-add"><c- g>add</c-></a>(<a class="idl-code" data-link-type="interface" href="https://webidl.spec.whatwg.org/#idl-DOMString" id="ref-for-idl-DOMString①"><c- b>DOMString</c-></a> <dfn class="dfn-paneled idl-code" data-dfn-for="CustomStateSet/add(value)" data-dfn-type="argument" data-export id="dom-customstates