Have you ever created a CMS or a browser? Or created a website? The demos shown require scripts to be written, e.g., as modules, to illustrate each example show.
That script has to be made compatible and optimized with the CMS or in this case the browser itself. This will include ensuring the the CSS will also work with the CMS or the browser.
If you ever created a website, you will find for example that the same script will not always work completely with various browsers There might be nuanced difference even among the more common browsers that adhere to the latest standards HTML. Ir you use a browser compatibility site to view your creation, what you may view based on the "same module script" will have nuanced differences if you view them in Firefox, Safari, Camino, Opera, Chrome, Internet Explorer, etc.
Usually, the most notorious and problematic with the resulting layout presentation are the various versions of Internet Explorer because Microsoft deliberately tweaked what it used to serve as html.
If you join any template sites, what you will find are problems ecountered by users of the template relevant to the resulting layout of the created template with respect to the various browsers.
What the developer of the template must then do is optimize the script to find a compromise that would then at least provide a lyout that would be decent for the more commonly used browsers and htheir various versions. This may include all sorts of tweaking, e.g., some Java scripts integerated to instruct --
"if an IE6 is used, ignore certain CSS specifications and use these alternative CSS"
For demos like this, for the masses, who simply view stuff. and not aware of all the hopps must go through to view an intented optimize layout, it must select a version of a specific browser. And optimize the script and CSS specifications of that module for that browser.
At this stage, Apple cannot force the browser creators to ensure that the various versions of the browsers still in existence to upgrade or modify their browser scripts to be optimized for the html5 demos created by Apple.
[A good non-Apple example of this is Google's You Tube. Since it is very time consuming and very difficult to ensure that the You Tube videos with all existing versions of the more common browsers, it opted to select speicific versions of browsers that are actively supported by the creator. Thus, Google decided not to support IE6 (not sure if there are older IE that it does not support for the You Tube videis. If you have an old computer or using older versions of certain browsers, you would have seen a warning months ago that Google You Tube will no longer function properly with certain browser versions,]
Since Apple owns Safari, it then opted to optimize it for specific versions of Safari. For example, some of the modules may use certain version of QuickTime to work for videos (perhaps as default) if your computer does not have certain applications required by the script.
I have Safari in my old computer but the demo requires a later version of QuickTime that in turn was optimized for certain versions of the OS for Mac, Windows, Linux, Chrome, etc. The OS of my old computer is below the minimum requirement, so I can download but not be able the compatible QuickTime required. All the other applications used must also be the appropriate version to work, Therefore, I cannot use my old version of Safari to view the Demo.