Which CSS selectors are the most expensive for the browser?

Selectors with high specificity and complex structure are the most expensive for the browser, especially:

• Descendant selectors (e.g., div ul li a), which cause the browser to traverse the DOM tree deeply.
• Universal selectors (*), which apply to all elements.
• Attribute selectors with partial matches (e.g., [class*="button"]), as they require checking each element.

For optimization, it is recommended to use classes and IDs, as ID selectors (#id) are the fastest, and to avoid overly nested selectors.