Silverlight is a free plug-in for web-based streaming media and some graphics, multimedia and animation applications. For many uses it is a Web development alternative for Adobe Flash. The most common use of Silverlight today is it’s requirement for streaming videos on Netflix. This can be problematic due to compatibility and support options for various Web browsers.
Safari doesn’t commonly have problems with Silverlight but it happens. As with Flash, if you get error messages that it has failed, restart your browser. If the problem persists, go to Microsoft’s Silverlight Installation page and install the most recent version available. If that fails and the problem continues, try uninstalling Silverlight completely before reinstalling it. Instructions for uninstalling on a Mac are here: Removing Silverlight Controls on a Macintosh. At Microsoft’s Silverlight installation page you can verify if you have Silverlight currently installed, or what version is installed, as well as determine if your Mac device is compatible with Silverlight.
Microsoft has announced discontinued support for Silverlight as of 2021. To that end, Netflix is planning to move away from Silverlight to HTML5, but this will require broad browser support (from Apple, Chrome, Microsoft, etc.) support for the new HTML5 features collectively known as “HTML5 Premium Video Extensions.” Timelines for this support implementations has not been announced yet by the World Wide Web Consortium or Netflix, but stay tuned!
You can get more information on Microsoft Silverlight here.