So, my limited understanding of the matter and 1:00 in the morning fogginess has boiled this information down to this: Basically, Apple had a program to port to Windows ten odd years ago, in the event that they chose to port their software to Windows, the project was dead as far as anyone knew for the last decade, but then they seem to have used it to port Safari? And there is a slight possibility that this tool may be made available to developers? If so, this sounds awfully interesting. We'll have to see where this goes.
It's all a little more complicated than that.
"Yellow Box" was a real and shipping product, though not under that name. Back when Apple was NeXT Inc, NeXT sold a product called (something like) OpenStep for NT, implementing their OpenStep API (native to NEXTSTEP) on NT. This eventually formed the base of the NT version of WebObjects. If you can find a copy of the NT version of WebObjects, then you can do old fashioned OpenStep programming with it.
When NeXT "bought" Apple for minus-several-zillion-dollars, the next version of NEXTSTEP became "Rhapsody", an operating system that comprised of the OpenStep API (like older NEXTSTEPs) plus a kind of virtual Mac where the upper layers of Mac OS ran inside a box to provide compatibility with older applications. The native (OpenStep) API was called YellowBox, the Mac OS compatibility environment was, IIRC, BlueBox, and a rumoured third environment was a Windows compatibility layer called RedBox, the exact details of which are, to date, unknown.
For consistency, the OpenStep on Windows product was renamed "YellowBox" in all the comments made at the time, but while WebObjects was sold for a little longer (as WebObjects, with little or no mention of the underlying OpenStep/YellowBox development environment), the entire "OpenStep on NT" concept was massively deprecated and the NT product, ultimately, dropped.
Rhapsody was well received at the time, but it was also widely felt that BlueBox was not an acceptable means of running older applications. Had it replaced Mac OS, it seems likely that most older applications would never have been ported to the modern APIs, and users would have spent most of their time in BlueBox with most of their applications, with only a slow trickle of YellowBox (OpenStep) apps being developed. The YellowBox APIs couldn't have been more different to the Mac OS APIs, and required an entirely different language and approach to use them. Developers were fairly unhappy, particularly those who had been loyal to Apple in the past and felt they'd have to throw pretty much everything out to produce native applications. Rhapsody was commercially released, in a Mac only form (Rhapsody's public betas had been available for both PC and Mac), in 1999 as "Mac OS X Server 1.0" but was barely marketed.
Apple then revamped the system. They used a compatibility layer for Quicktime on Windows as the basis of a new native API for the NEXTSTEP based operating system line. This supported a great deal of the Mac OS's API, and made porting existing Mac OS apps much easier. This was named Carbon, with YellowBox renamed to Cocoa, and BlueBox and renamed to Classic. This is what makes up the OS today, and has done so on the Mac OS X Beta and Mac OS X 10.x versions of the operating system.
So the long and the short is:
- "YellowBox" (eg OpenStep or Cocoa for Windows) has existed for a long time. It's not a rumour. It is a product that ultimately disappeared from the catalogs with few people even noticing
- Apple has always had compatibility layers written to allow it to port apps to Windows, the aforementioned Quicktime origins for Carbon being an obvious example
- This story doesn't really mean a lot. The fact Safari has some of this code doesn't mean Apple is about to release a public development environment for cross-platform apps. Quicktime for Windows and WebObjects both had/have elements allowing this, but that's not how they were sold. Anyone trying to develop using what Apple has publicly released up until now will suffer a lack of documentation and integrated "universal binary" tools.