CSS Custom Selectors (Level 4)

 CSS Custom Selectors, also known as Level 4 Selectors, introduce new features that provide more powerful and concise ways to target elements in CSS. These features include the :is(), :where(), and :not() pseudo-classes. Let's explore each of them:

1. :is() Pseudo-class:

   • The :is() pseudo-class allows you to specify multiple selectors within a single rule, reducing redundancy in your CSS code.
   • It accepts a comma-separated list of selectors and applies the styles if any of the selectors match the element.
   • This is particularly useful when you have several selectors that share the same styles.
   • Example:
   • 
     
   /* Instead of */ article h1, article h2, article h3, article h4 { font-family: 'Arial', sans-serif; } /* You can use */ article :is(h1, h2, h3, h4) { font-family: 'Arial', sans-serif; }
   
   

2. :where() Pseudo-class:

   • The :where() pseudo-class works similarly to :is(), but it accepts any number of selectors as arguments.
   • It behaves as if all the selectors inside :where() were grouped together without any additional specificity.
   • This can be useful for resetting or normalizing styles across multiple elements.
   • Example:
   • 
     
   • /* Instead of */ article h1, article h2, article h3, article h4 { margin: 0; padding: 0; } /* You can use */ :where(article h1, article h2, article h3, article h4) { margin: 0; padding: 0; }
   
   

3. :not() Pseudo-class:

   • The :not() pseudo-class allows you to select elements that do not match a given selector.
   • It accepts a single argument, which is a selector representing the elements to be excluded.
   • This is useful for targeting specific elements while excluding others.
   • Example:
   • 
     
   • /* Select all paragraphs except those with a class of "special" */ p:not(.special) { color: #333; }

4. :has() Pseudo-class:

   • The :has() pseudo-class allows you to select elements that contain specific descendants, even if those descendants are not direct children.
   • It accepts a single argument, which is a selector representing the descendants to be checked for.
   • This pseudo-class is particularly useful for styling parent elements based on the presence or characteristics of their descendants.
   • Example:
   • 
     
   • /* Style list items that contain an anchor element */ li:has(a) { background-color: lightblue; }
   
   

5. :matches() Pseudo-class:

   • The :matches() pseudo-class, also known as :any(), allows you to select elements that match any of the specified selectors.
   • It behaves similarly to :is(), but it is more versatile as it can be nested within other selectors.
   • Example:
   • 
     
   • /* Select headings that are either h1, h2, or h3 */ h1:matches(h1, h2, h3) { font-weight: bold; }
   
   

6. ::part() Pseudo-element:

   • The ::part() pseudo-element allows you to style specific parts of a shadow DOM element.
   • It targets the internal parts defined in the shadow DOM of a web component, providing better encapsulation and styling control.
   • Example:
   • 
     
   • /* Style the scrollbar thumb of a custom element */ ::part(scrollbar-thumb) { background-color: #ccc; }

These additional features of CSS Custom Selectors (Level 4) further enhance the power and flexibility of CSS selectors, enabling developers to write more concise, expressive, and maintainable CSS code. As with any new CSS features, it's essential to check for browser compatibility and consider fallbacks or polyfills for older browsers if necessary.