Performance implications of certain CSS Selectors are not specific to a certain JavaScript Library like Prototype. I recently blogged about the internals of CSS Selectors in jQuery. The same holds true for every JavaScript library that offers CSS Selectors. Certain lookups can be done by using the native browser functions like getElementById or getElementsByTagName. Lookups by class name are not natively supported in IE and are therefore implemented in JavaScript by iterating through all elements in the DOM – which is obviously much slower than a native implementation. Expensive Prototype CSS Selector Message was edited by: