Opera has released a public build (development snapshot) with support for HTML5 WebSockets, which will be built into Opera 10.70.
With this announcement, Opera has joined the ranks of Firefox, Chrome and Safari in WebSockets support. Now, IE is the only major browser left that has yet to support HTML5 WebSockets.
HTML5 Web Sockets allow a web application to create bidirectional TCP (Transmission Control Protocol) connections from the browser to a server. These Web sockets, being bidirectional, imply that the server can now push data to the browser.
Prior to Web Sockets, web apps such as web Instant Messaging (Meebo, EBuddy, etc) often made use of Ajax techniques, specifically XMLHttpRequest objects or invisible to repeatedly poll the web server for updates. Each poll would open a short-lived connection that will be closed when the server responses to the poll request – often, the response would contain little or no content. This results significant overheads for both processing and bandwidth.
(Please refer to my short writeup on HTML5 for more details.)