JavaScript has inconsistent and buggy behavior across different browsers due to variations in rendering engines like Trident, WebKit, and Gecko, and JavaScript engines like Spider Monkey, V8, SquirrelFish, and Chakra. JavaScript was created in the 1990s and introduced the concept of Ajax in 2005, allowing dynamic web page updates without reloading the entire page. This document provides resources on how browsers work and the history of Ajax.