CSS selectors, akin to SQL statements in backend development, play a crucial role in frontend development by being used extensively in JavaScript and offering opportunities for optimization due to their declarative nature. Modern web applications utilize thousands of selectors, requiring browsers to employ advanced techniques to efficiently match and process these selectors, such as WebKit's RuleSet. SelectorSet, a JavaScript implementation, enhances this by quickly indexing selectors using keys derived from significant parts of the selectors, optimizing the matching process. Document delegated events in jQuery, particularly through functions like $.fn.on, benefit from this optimization, allowing event handlers to trigger for new elements without reloading the page, a necessity in patterns like pjax. While extensive use of document delegated event handlers can be controversial due to potential performance drawbacks in deeply nested DOMs, the SelectorSet technique can significantly enhance performance. Despite these advancements, the hope remains for browsers to natively support efficient processing of CSS selectors and declarative event handlers, as the current solutions are workarounds for existing limitations.