Microsoft today released Silverlight 5, which could be the last major version of Microsoft's .... Microsoft's ... Hhm. Just what the heck is Silverlight?

According to Microsoft's web site, Silverlight is many things. It's "a powerful development tool for creating engaging, interactive user experiences for Web and mobile applications." Which sounds simple enough until you read the next line, "It's a free plug-in, powered by the .NET framework and compatible with multiple browsers, devices and operating systems." Which it is, of course, unless you're talking about the framework that's on Windows Phone, which is most decidedly not a plug-in.

In other words, Silverlight is Microsoft's version of the old SNL skit for Shimmer floor wax. "It's a floor wax! No, it's a dessert topping!"

Which may explain why they're (probably) killing it off.

That is, Silverlight has grown out of control. What started off as something very simple--a pretty bald-faced attempt to copy Adobe's successful Flash platform--has mutated beyond anyone's expectations.  It started as a way to host a subset of the .NET Frameworks within a web browser, using the now-derided browser plug-in model, bringing advanced interactive features to the web. Over time, Silverlight grew and picked up capabilities--including more .NET features and better programming language support--so much so that the differences between it and the similar but more desktop-focused Windows Presentation Foundation (WPF) became somewhat blurred.

As noted above, Microsoft also confusingly uses Silverlight for its main application framework on Windows Phone, and not some mobile version of WPF. It doesn't help, further, that the Silverlight version used on Windows Phone is always one version behind one offered to web developers.

Ah, but Silverlight 5. What's going on here?

This version was first announced a year ago and originally expected in the second half of 2011. Due to the huge silence between the April 2011 beta and now, speculation arose that Microsoft was considering killing off Silverlight, perhaps in lieu of yet another similar framework, the Windows Runtime (WinRT) that's being created for Windows 8. Or just because HTML 5 happened. 

My Windows Weekly co-host Mary Jo Foley is now reporting that this may indeed be the last version of the framework, whatever the reason. Microsoft, as is the case so much these days, is silent on the topic.

To be clear, the Silverlight 5 that's being released today is the web browser plug-in, not the floor wax, er ah, the Windows Phone framework. It's compatible with Chrome, Firefox, Internet Explorer, and Safari. And I can't think of a single reason why you'd want it. I really can't.

Note: This post original noted that Silverlight 5 was "originally expected in the first half of 2010." That should have read, "the second half of 2011." I've updated the text above. --Paul