Added optional selector (#28540)

* Added optional selector

Added optional selector

* Update index.md

guide/english/css/selectors/pseudo/optional/index.md

@@ -0,0 +1,23 @@
+---
+title: Optional
+---
+## Optional
+
+The CSS :optional selector changes the style of an input element if the `required` tag is not used. 
+The :optional selector can be used on the  input, select and textarea element
+
+In the example below, the required field will be blue and the optional field red
+
+```css
+input {
+  background-color: blue;
+}
+
+input:optional {
+  background-color: red;
+}
+```
+
+#### More Information:
+* [MDN Web Docs](https://developer.mozilla.org/en-US/docs/Web/CSS/:optional)
+* [W3 Schools](https://www.w3schools.com/cssref/sel_optional.asp)