When considering data transmission techniques, you must take into account several factors: feature set, compatibility, performance, and direction (to or from the server). When considering data formats, the only scale you need for comparison is speed.
When Ajax first became popular, XML was the data format of choice. It had many things going for it: extreme interoperability (with excellent support on both the server side and the client side), strict formatting, and easy validation. JSON hadn’t been formalized yet as an interchange format, and almost every language used on servers had a library available for working with XML.
Here is an example of our list of users encoded as XML:
<?xml version="1.0" encoding='UTF-8'?> <users total="4"> <user id="1"> <username>alice</username> <realname>Alice ...