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The 45.3MB private beta, labeled Safari 5.1 Developer Preview, will deliver improved graphics performance on Windows, media caching for audio and video data for web apps using the HTML5 application cache, and a new process architecture that separates the browser's rendering process from its application process, making Safari more responsive and stable.
Along these same lines, Safari 5.1 (on Snow Leopard and Lion) will run plug-ins within their own process, which should improve both security and stability.
For developers, the upcoming release introduces support for full-screen web content and support for keyboard behavior control by introducing several new methods to the Element and Document classes, according to people familiar with the software.
The forthcoming release will also offer developers new ways to customize and interact with Safari through Safari Extensions. For instance, developers will be able to receive event notifications when a user opens a window or tab, or when a window or tab has gone inactive.
Similarly, developers will gain access to a feature called Extension Menus, which will allow them to deliver interactive and customizable menus from within the Safari interface, as well as access to throw users into and out of the browser's Reader feature.
Another Extensions API called Extensions Popover should also allow developers to present users with large chunks of customized content that won't interfere with existing web content loaded in a tab or browser window.
Additionally, Safari 5.1 will roll out support for Web Open Font Format (WOFF), Mathematical Markup Language (MathML), CSS3 Auto-hyphenation, CSS3 Text Emphasis, CSS3 Vertical Text, and CSS3 Transitions and Animations.