First stab at definition of "disabled"

Author: kasperisager

pages/glossary/disabled.md

@@ -3,4 +3,31 @@ title: Disabled
 key: disabled
 ---
 
-TODO
+An element that is normally [perceivable][] and [operable][], but has been rendered [inoperable][operable] either temporarily or permanently, is considered disabled. While there exists several techniques through which elements can be disabled, the most typical are:
+
+1. The [`disabled`][disabled] attribute. The presence of this attribute, regardless of its value, either
+
+    - on a [`button`][button], [`input`][input], [`select`][select], [`textarea`][textarea], or [form-associated custom element][]; or
+    - on a [`fieldset`][fieldset] element
+    
+    will disable the element itself if it is not a [`fieldset`][fieldset] or, if it is, will disable any [descendants][descendant] of the element, excluding those that are [descendants][descendant] of the first [`legend`][legend] [child][] of the element.
+    
+    **Note:** When the [`disabled`][disabled] attribute is specified on a [`fieldset`][fieldset] element, [shadow-including descendants][shadow-including descendant] are **not** disabled.
+
+2. The [`aria-disabled`][aria-disabled] attribute. The presence of this attribute with a value of `true` will semantically disable an element and its [focusable][] [shadow-including descendants][shadow-including descendant].
+
+[operable]: https://www.w3.org/TR/wai-aria/#dfn-operable
+[perceivable]: https://www.w3.org/TR/wai-aria/#dfn-perceivable
+[aria-disabled]: https://www.w3.org/TR/wai-aria/#aria-disabled
+[disabled]: https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#attr-fe-disabled
+[form-associated custom element]: https://html.spec.whatwg.org/multipage/custom-elements.html#form-associated-custom-element
+[button]: https://html.spec.whatwg.org/multipage/form-elements.html#the-button-element
+[input]: https://html.spec.whatwg.org/multipage/input.html#the-input-element
+[select]: https://html.spec.whatwg.org/multipage/form-elements.html#the-select-element
+[textarea]: https://html.spec.whatwg.org/multipage/form-elements.html#the-textarea-element
+[fieldset]: https://html.spec.whatwg.org/multipage/form-elements.html#the-fieldset-element
+[descendant]: https://dom.spec.whatwg.org/#concept-tree-descendant
+[legend]: https://html.spec.whatwg.org/multipage/form-elements.html#the-legend-element
+[child]: https://dom.spec.whatwg.org/#concept-tree-child
+[shadow-including descendant]: https://dom.spec.whatwg.org/#concept-shadow-including-descendant
+[focusable]: https://html.spec.whatwg.org/multipage/interaction.html#focusable-area