fix(css): improve :defined pseudo-class example (#35371)

* fix(css): improve :defined pseudo-class example

* Apply suggestions from code review

Co-authored-by: Chris Mills <[email protected]>

* address review comments

* small tweak

---------

Co-authored-by: Chris Mills <[email protected]>

files/en-us/web/css/_colon_defined/index.md

@@ -7,7 +7,7 @@ browser-compat: css.selectors.defined
 
 {{CSSRef}}
 
-The **`:defined`** [CSS](/en-US/docs/Web/CSS) [pseudo-class](/en-US/docs/Web/CSS/Pseudo-classes) represents any element that has been defined. This includes any standard element built in to the browser, and custom elements that have been successfully defined (i.e. with the {{domxref("CustomElementRegistry.define()")}} method).
+The **`:defined`** [CSS](/en-US/docs/Web/CSS) [pseudo-class](/en-US/docs/Web/CSS/Pseudo-classes) represents any element that has been defined. This includes any standard element built into the browser and custom elements that have been successfully defined (i.e. with the {{domxref("CustomElementRegistry.define()")}} method).
 
 ```css
 /* Selects any defined element */
@@ -33,65 +33,95 @@ simple-custom:defined {
 
 ### Hiding elements until they are defined
 
-The following snippets are taken from our [defined-pseudo-class](https://github.com/mdn/web-components-examples/tree/main/defined-pseudo-class) demo ([see it live also](https://mdn.github.io/web-components-examples/defined-pseudo-class/)).
+In this demo, we define a basic custom element named `<custom-element>` and use the `:not(:defined)` and `:defined` selectors to style the element before and after it is defined. This is useful if you have a complex custom element that takes a while to load into the page — you might want to hide instances of the element until the definition is complete so that you don't end up with flashes of ugly unstyled elements on the page.
 
-In this demo we define a very simple trivial custom element:
+#### HTML
 
-```js
-customElements.define(
-  "simple-custom",
-  class extends HTMLElement {
-    constructor() {
-      super();
-
-      let divElem = document.createElement("div");
-      divElem.textContent = this.getAttribute("text");
-
-      let shadowRoot = this.attachShadow({ mode: "open" }).appendChild(divElem);
-    }
-  },
-);
-```
-
-Then insert a copy of this element into the document, along with a standard `<p>`:
+The following HTML code uses the custom element but the element hasn't been defined yet. We also include a {{htmlelement("button")}} that will define the custom element when clicked, allowing you to see its state before and after definition.
 
 ```html
-<simple-custom text="Custom element example text"></simple-custom>
-
-<p>Standard paragraph example text</p>
+<custom-element>
+  <p>
+    Loaded content: Lorem ipsum tel sed tellus eiusmod tellus. Aenean. Semper
+    dolor sit nisi. Elit porttitor nisi sit vivamus.
+  </p>
+</custom-element>
+
+<button id="btn">define the <code>&lt;custom-element&gt;</code></button>
 ```
 
-In the CSS we first include the following rules:
+#### CSS
 
-```css
-/* Give the two elements distinctive backgrounds */
-p {
-  background: yellow;
+```css hidden
+custom-element {
+  display: block;
+  border: 5px dashed grey;
+  border-radius: 1rem;
+  height: 100px;
+  width: 400px;
+  padding: 1rem;
+  position: relative;
+  user-select: none;
 }
 
-simple-custom {
-  background: cyan;
+code {
+  background: #ccc;
 }
 
-/* Both the custom and the built-in element are given italic text */
-:defined {
-  font-style: italic;
+#btn {
+  margin-top: 1rem;
+  cursor: pointer;
 }
 ```
 
-Then provide the following two rules to hide any instances of our custom element that are not defined, and display instances that are defined as block level elements:
+In the following CSS, we use the `custom-element:not(:defined)` selector to select the element and color it grey while it is not defined and the `custom-element:defined` selector to select the element and color it black after it is defined.
 
 ```css
-simple-custom:not(:defined) {
-  display: none;
+custom-element:not(:defined) {
+  border-color: grey;
+  color: grey;
 }
 
-simple-custom:defined {
-  display: block;
+custom-element:defined {
+  background-color: wheat;
+  border-color: black;
+  color: black;
 }
+
+/* show loading message */
+custom-element:not(:defined)::before {
+  content: "Loading...";
+  position: absolute;
+  inset: 0 0 0 0;
+  align-content: center;
+  text-align: center;
+  font-size: 2rem;
+  background-color: white;
+  border-radius: 1rem;
+}
+
+/* remove the loading message */
+custom-element:defined::before {
+  content: "";
+}
+```
+
+We have also used the [`::before`](/en-US/docs/Web/CSS/::before) pseudo-element to show a "Loading..." overlay message until the element is defined. After definition, it is removed by setting the [`content`](/en-US/docs/Web/CSS/content) to an empty string.
+
+The following JavaScript has been used to define the custom element. To allow you to see the state of the custom element before and after definition we run the {{domxref("CustomElementRegistry.define", "define()")}} method when the button is clicked.
+
+```js
+const btn = document.querySelector("#btn");
+
+btn.addEventListener("click", () => {
+  customElements.define("custom-element", class extends HTMLElement {});
+  btn.remove();
+});
 ```
 
-This is useful if you have a complex custom element that takes a while to load into the page — you might want to hide instances of the element until definition is complete, so that you don't end up with flashes of ugly unstyled elements on the page
+#### Result
+
+{{EmbedLiveSample("Hiding elements until they are defined", "100%", "230")}}
 
 ## Specifications