Safari is built expressly for Apple's Mac OS X.

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The Firefox OS for mobile devices is built on Mozilla's "Boot to Gecko project" which unlocks many of the current limitations of web development on mobile, allowing HTML5 applications to access the underlying capabilities of a phone, previously only available to native applications.

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Built for the cloud, they run through a combination of the Chrome OS browser and Web apps.

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Profiles are built in for popular apps such as Microsoft Office, Media Player, QuickTime Player, browsers and Mac OS X.

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Apple also raided the iOS cupboard for Messages, an app whose Mac edition debuted as a public beta and is now built into OS X.

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Tizen has one particularly strong backer, Samsung, who built the reference device we play with in the video after the break, and whereas Firefox OS is destined only for the low-end, Tizen seems far more ambitious -- at least judging from the 720p resolution of this developer handset.

ENGADGET

With few exceptions, the most compelling Ultrabooks on offer there ( this, this, this and this, for example) were built with touchscreens to help make the most of Microsoft's upcoming, touch-friendly OS. Sure, you might be loathe to track fingerprints all over your 1080p display, or maybe you agree with Mr. Cook that tablets and notebooks are best kept separate.

ENGADGET: MacBook Air review (13-inch, mid 2012)