diff --git a/src/nwsapi.js b/src/nwsapi.js
index dd92116..f10ce66 100644
--- a/src/nwsapi.js
+++ b/src/nwsapi.js
@@ -1123,10 +1123,31 @@
                     ')){' + source + '}';
                   break;
                 case 'disabled':
-                  // https://www.w3.org/TR/html5/forms.html#enabling-and-disabling-form-controls:-the-disabled-attribute
-                  source = 'if((("form" in e||/^optgroup$/i.test(e.localName))&&"disabled" in e&&' +
-                    '(e.disabled===true||(n=s.ancestor("fieldset",e))&&(n=s.first("legend",n))&&!n.contains(e))' +
-                    ')){' + source + '}';
+                  // https://html.spec.whatwg.org/#enabling-and-disabling-form-controls:-the-disabled-attribute
+                  source = 'if((("form" in e||/^optgroup$/i.test(e.localName))&&"disabled" in e)){' +
+                    // F is true if any of the fieldset elements in the ancestry chain has the disabled attribute specified
+                    // L is true if the first legend element of the fieldset contains the element
+                    'var x=0,N=[],F=false,L=false;' +
+                    'if(!(/^(optgroup|option)$/i.test(e.localName))){' +
+                      'n=e.parentElement;' +
+                      'while(n){' +
+                        'if(n.localName=="fieldset"){' +
+                          'N[x++]=n;' +
+                          'if(n.disabled===true){' +
+                            'F=true;' +
+                            'break;' +
+                          '}' +
+                        '}' +
+                        'n=n.parentElement;' +
+                      '}' +
+                      'for(var x=0;x<N.length;x++){' +
+                        'if((n=s.first("legend",N[x]))&&n.contains(e)){' +
+                          'L=true;' +
+                          'break;' +
+                        '}' +
+                      '}' +
+                    '}' +
+                    'if(e.disabled===true||(F&&!L)){' + source + '}}';
                   break;
                 case 'read-only':
                   source =
diff --git a/test/w3c/html/semantics/selectors/pseudo-classes/disabled.html b/test/w3c/html/semantics/selectors/pseudo-classes/disabled.html
index 8808675..142a12d 100644
--- a/test/w3c/html/semantics/selectors/pseudo-classes/disabled.html
+++ b/test/w3c/html/semantics/selectors/pseudo-classes/disabled.html
@@ -57,4 +57,24 @@
   var input = document.createElement("input");
   input.setAttribute("disabled", "disabled");
   testSelectorIdsMatch(":disabled", ["button1", "input2", "select2", "optgroup2", "option2", "textarea2", "fieldset2", "clubname", "clubnum"], "':disabled' should not match elements not in the document");
+
+  var fieldset = document.createElement("fieldset");
+  fieldset.id = "fieldset_nested";
+  fieldset.innerHTML = `
+    <input id=input_nested>
+    <button id=button_nested>button nested</button>
+    <select id=select_nested>
+      <optgroup label="options" id=optgroup_nested>
+        <option value="options" id=option_nested>option nested</option>
+      </optgroup>
+    </select>
+    <textarea id=textarea_nested>textarea nested</textarea>
+    <object id=object_nested></object>
+    <output id=output_nested></output>
+    <fieldset id=fieldset_nested2>
+      <input id=input_nested2>
+    </fieldset>
+  `;
+  document.getElementById("fieldset2").appendChild(fieldset);
+  testSelectorIdsMatch("#fieldset2 :disabled", ["clubname", "clubnum", "fieldset_nested", "input_nested", "button_nested", "select_nested", "textarea_nested", "fieldset_nested2", "input_nested2"], "':disabled' should match elements that are appended to a disabled fieldset dynamically");
 </script>