One of the scenarios Microsoft aimed at Silverlight was applications like LOB (Line Of Business). This is seriously affecting this area - it is delivered via the Internet, and its performance is the same in every supported browser, I don’t need to encode different versions for different browsers.
HTML5 is still an undisclosed and evolving standard, and each browser has different bits in different browsers. I have nothing against HTML5, but as soon as it becomes final (if it ever appears), you will get more of the same as what has already happened - different browser developers implement it a little differently, therefore, there will be differences between browsers that you, as a developer, will have to make a discount.
With the release of Silverlight 4, MS has made great strides, and it tosses up the ass even more, providing access to webcams, printers, ending with a browser, accessing the file system if it is trusted, etc. This makes it even more desktop-like, for example, you will never achieve this with HTML 5 and jQuery, they remain strictly a browser / web technology.
Silverlight is haunted and has a large (and growing) takeover. What you should be thinking about is that Silverlight is starting to become a fairly mature technology, while HTML5 has not even been born.
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