Monday's debut of Apple's Safari 5 Web browser was one of the Worldwide Developers Conference rumors that preceded the event over the weekend, and it surprised many Steve Jobs prognosticators that he left the stage without announcing it. Then came an Apple press release on, but no browser. Then the press release disappeared, apparently a second stumble following a rare trip-up for Steve Jobs' tightly controlled message machine during the iPhone 4 announcement, and shortly after that the browser made its debut.

Available for Windows and Mac, Safari 5 includes several new features and performance enhancements, the biggest of which has been announced but not activated. The new Safari Developer Program marks the advent of digitally signed add-ons for Safari.

The new Reader feature in Safari streamlines both single and multi-page stories into a more legible format. (Credit: Screenshot by Seth Rosenblatt/CNET)

Currently, only Safari for Mac supports add-ons, but the implication from the Safari Web site is that the new extension format will be cross-platform because they will be built on HTML5, CSS3, and JavaScript. Apple is clearly trying to get some of their newly re-christened iOS mojo to rub off on the desktop Safari since the new add-ons will be "sandboxed, signed with a digital certificate from Apple and run solely in the browser," according to the pulled Safari 5 press release.

Given the language used to describe the program and Apple's controlled iOS marketplace, the Safari Extensions so far sounds like an attempt to extend the iOS's philosophy to the browser, although it could wind up being nothing more than the Safari version of Mozilla add-ons site.

Safari 5 also includes Safari Reader for streamlining articles, a new add-on network, dramatic performance improvements, and support for Bing search. Safari Reader is designed to streamline the browser to maximize reading space when viewing single and multipage articles. A new Reader icon in the location bar will toggle the feature, and there are mouse-over options to enlarge the story, print it, or share it via e-mail.

HTML5 support in the new version of Safari doesn't do any major trailblazing, although it does add significant support for the next generation of HTML code. Safari now supports HTML5-based full-screen video playback, video closed captioning, geolocation, drag and drop, forms validation, HTML5 Ruby, EventSource, and WebSocket. Mozilla Firefox, Google Chrome, and Opera all support these features either in their stable releases or in generally functional development versions, leaving Internet Explorer with support only in IE 9's stripped-down pre-alpha builds.

Apple is claiming major improvements to Safari's JavaScript engine Nitro, with the version in Safari 5 running 30 percent faster than Safari 4. Apple says it also benchmarked Safari 5 to be twice as fast as Firefox 3.6, and 3 percent faster than Chrome 5.0. Safari 5 also loads Web pages faster by using DNS prefetching.

Updated at 10:30 p.m. to clarify headline.