MSSL meaning in Computing ?

Microsoft Silverlight is a discontinued application framework designed for writing and running rich web applications, similar to Adobe's runtime, Adobe Flash. A plugin for Silverlight is still available for a very small number of browsers. While early versions of Silverlight focused on streaming media, later versions supported multimedia, graphics, and animation, and gave support to developers for CLI languages and development tools. Silverlight was one of the two application development platforms for Windows Phone, but web pages using Silverlight did not run on the Windows Phone or Windows Mobile versions of Internet Explorer, as there was no Silverlight plugin for Internet Explorer on those platforms.

Microsoft terminated support for Silverlight on Internet Explorer 11 (the last remaining web browser still supporting Silverlight) on October 12, 2021. It is supported on Windows 8.1 and later and Windows Server 2012 and later, and e.g. Windows Embedded POSReady 7 and Windows Thin PC, while e.g. Windows 7 and earlier do not get security updates.