Why IE 8 won't use the WebKit rendering engine

Despite the fact that many developers would welcome more rendering-engine compatibility, anyone expecting Microsoft to switch out its "Trident" engine and replace it with WebKit in Internet Explorer (IE) -- especially in IE 8 -- is misguided.

Comments this week by CEO Steve Ballmer set off speculation about Microsoft's WebKit plans. Responding to a question during an appearance for the Australian development community, Ballmer said Microsoft could "from time to time take a look at" WebKit, the open-source rendering engine at the heart of the Apple Safari and Google Chrome browsers.

I can see a number of reasons Microsoft won't be adopting WebKit any time soon -- and not simply because introducing an open-source engine into a closed-source browser would set off many Softies' alarm bells.

Just to be clear: I am not defending Microsoft's decision to stick with Trident; I'm explaining what I believe to be the company's philosophy around that decision. Here's why I don't see Microsoft switching allegiances to WebKit -- at least not in the near term:

1. IE 8 is almost done, folks. Beta 2, which Microsoft released in August,  is expected to be the final public beta. The final IE 8 is due before the end of the year -- which I've interpreted and heard  from sources meant "end of calendar 2008," but I hear Microsoft might be claiming to be "end of fiscal 2009," which would mean in the first half of 2009. In either case, Microsoft is not going to rip out Trident and replace it with WebKit at this late date.

2. Microsoft is pushing test cases, not rendering engines, as its best way to help ease developers' compatibility headaches. Microsoft is releasing a growing set of test cases specific to IE as its preferred strategy for helping developers make sure their pages and apps will work with Microsoft's browser.

3. Microsoft is not "consider(ing) adopting WebKit for Internet Explorer." As my ZDNet blogging colleague Larry Dignan explained, all Ballmer actually said this week was WebKit was interesting and worth taking a look at. Microsoft looks at lots of competitive technologies on a regular basis. "Looking at" doesn't equate to "adopt."

I don't know whether changing the IE 8 rendering engine from Trident to WebKit would wreak havoc on existing Web sites/apps that have been designed to work with current versions of IE. (Anyone out there have any thoughts on this?)

Perhaps there's a way Microsoft could do this that wouldn't be overly disruptive to the majority of users. One commentator, responding to a January 21 post on "Compatibility and IE 8" on the Microsoft IE 8 blog suggested Microsoft introduce WebKit as a plug-in:

"Why not make the rendering engine a plugin, develop the engine as open-source but let the rest of the browser closed-source. Each browser will have its own proprietary interface, unique set of features, but at least the rendering engine will be essentially the same."

Would it be feasible for Microsoft make WebKit a plug-in to IE? Would the company consider doing so with IE 9 (which, if Microsoft sticks to its current schedule, would be released in 2011 or so)?  Developers: What do you think Microsoft can, should and will do, regarding WebKit and IE?