I think Microsoft has a bigger interest in an open web then Apple does. Actually, I don't see how Apple's business model fits in any shape or form into the idea of an open web.
Apple wants control over the ways people develop software (hi 3.1.1!) and ensure that software runs best on iPhone OS only, if it even runs anywhere else. That way they can keep racking in the iPhone OS devices' and subscriptions' money.
Obviously Apple is also a platform vendor and would rather have their platform have the best web experience by introducing proprietary Apple OS X / iPhone only tech on the web if possible. The thing is they are in no position to make this happen.
Microsoft has a better shot at this. When people view the web it is usually through a Microsoft product (~90% on a Microsoft OS). Silverlight supports Mac OS X, but I think this is more to entice developers to the platform then anything else. There is no
business case for it other then appeasing the demand for cross-platform development. Microsoft is a platform vendor, and as a platform vendor, a business case for cross-platform development (which means improving your competitor's platforms) is almost nonsensical.
If Silverlight ever gets popular I'm sure the Mac OS X version will go the way of the dodo, probably with Microsoft citing "lack of demand" as typical. This will lead to a greatly degraded web experience for everyone not using Windows. Which will make Windows
more appealing. Which arguably was the whole point of the experiment.