fix(css): add :open page

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

@@ -0,0 +1,90 @@
+---
+title: ":open"
+slug: Web/CSS/:open
+page-type: css-pseudo-class
+browser-compat: css.selectors.open
+---
+
+{{CSSRef}}
+
+The **`:open`** [CSS](/en-US/docs/Web/CSS) [pseudo-class](/en-US/docs/Web/CSS/Pseudo-classes) selects elements in the open state. It works only on elements that have both open and closed states. For example, it matches {{HTMLElement("details")}} and {{HTMLElement("dialog")}} when they are in their open state, and matches {{HTMLElement("select")}} and {{HTMLElement("input")}} when they are in modes which have a picker and the picker is showing.
+
+## Syntax
+
+```css
+:open {
+  /* ... */
+}
+```
+
+## Examples
+
+This example demonstrates some of the HTML elements that have an open state.
+
+### CSS
+
+```css
+details:open > summary {
+  background-color: pink;
+}
+
+:is(select, input):open {
+  background-color: pink;
+}
+```
+
+```css hidden
+@supports not selector(:open) {
+  body::before {
+    content: "Your browser doesn't support :open selector.";
+    background-color: wheat;
+    display: block;
+    width: 100%;
+    text-align: center;
+  }
+
+  body > * {
+    display: none;
+  }
+}
+```
+
+### HTML
+
+```html
+<details>
+  <summary>Details</summary>
+  Lorem ipsum dolor sit amet, consectetur adipiscing elit. In pulvinar dapibus
+  lacus, sit amet finibus lectus mollis eu. Nullam quis orci dictum, porta lacus
+  et, cursus nunc. Aenean pulvinar imperdiet neque fermentum facilisis. Nulla
+  facilisi. Curabitur vitae sapien ut nunc pulvinar semper vitae vitae nisi.
+</details>
+<hr />
+
+<label for="pet-select">Choose a pet:</label>
+<select id="pet-select">
+  <option value="dog">Dog</option>
+  <option value="cat">Cat</option>
+  <option value="hamster">Hamster</option>
+</select>
+<hr />
+
+<label for="start">Start date:</label>
+<input type="date" id="start" />
+```
+
+### Result
+
+{{EmbedLiveSample("Examples", 300, 200)}}
+
+## Specifications
+
+{{Specifications}}
+
+## Browser compatibility
+
+{{Compat}}
+
+## See also
+
+- {{Cssxref(":modal")}}