AHAH is a very simple technique for dynamically updating web pages using JavaScript . It involves using XMLHTTPRequest  to retrieve (X)HTML  fragments which are then inserted directly into the web page, whence they can be styled using CSS. it is a subset with some very specific and useful properties, such as the lack of custom XML schemas. AHAH can trivially reuse existing HTML pages, avoiding the need for a custom web service. All data transport is done via browser-friendly HTML, easing debugging and testing. The HTML is designed to be directly embedded in the page’s DOM, eliminating the need for parsing.