[17] Internally, even proponents of the technology thought Extensible Application Markup Language as a concept was a bad idea from the start.

[19] In July 2015, a Microsoft blog post clarified that, "… we encourage companies that are using Silverlight for media to begin the transition to DASH/MSE/CENC/EME based designs".

[34] A free software implementation (now abandoned)[35] named Moonlight, developed by Novell in cooperation with Microsoft, was released to bring Silverlight version 1 and 2 functionality to Linux, FreeBSD, and other open source platforms, although some Linux distributions did not include it, citing redistribution and patent concerns.

The following table presents an availability and compatibility matrix of Silverlight versions for various operating systems and web browsers.

Moonlight is available for the major Linux distributions, with support for Firefox, Konqueror, and Opera browsers, provided it was obtained through Novell.

Pipelight requires browser support for NPAPI plugins, which newer versions of Firefox, Chrome, and Opera have dropped.

[59] Silverlight was not available on Android or iOS, the most prevalent operating systems on the mobile market.

[60] Nokia announced plans to make Silverlight for Mobile available for S60 on Symbian OS, as well as for Series 40 devices and Nokia internet tablets[61][62] (while it later sold the business to Microsoft and now sells Android tablets and will sell Alcatel-Lucent branded smartphones).

[66] An April 2007 PC World report, suggested that Microsoft intended to release certain parts of Silverlight source code as open source software,[67] but a week later Sam Ramji, director of platform technology strategy at Microsoft, contradicted the rumors by confirming that the company had no plans to open Silverlight.

Advocates of free software were also concerned Silverlight could be another example of Microsoft's embrace, extend, and extinguish strategy.

The Mono Team abandoned development of Moonlight, a free and open-source implementation of both the Silverlight 1 and 2 runtimes.

[76] Development was discontinued in 2012 due to the poor acceptance of Silverlight and the restrictions imposed by Microsoft.

[77] The project had been officially supported by Microsoft which,[53] under an agreement with Novell, made not-publicly-available additional specifications, access to the Silverlight Base Class Library APIs, binary codecs and test cases available to the Mono team.

[54][78] The "covenant" under which Novell was granted this exclusive access also specified conditions incompatible with the licensing that covers most free and open source software.

Although Microsoft was officially collaborating on the Moonlight project, Bruce Chizen, CEO of Adobe Systems, which sells the competing proprietary Flash platform, questioned "the commitment of Microsoft to keep the Silverlight platform compatible with other OS besides Windows".

In 2007, California and several other U.S. states asked a district judge to extend most of Microsoft's antitrust case settlement for another five years,[81] citing "a number of concerns, including the fear that Microsoft could use the next version of Windows to 'tilt the playing field' toward Silverlight, its new Adobe Flash competitor," says a Seattle Post-Intelligencer article.

The final judgment on the motion extended the settlement two years, to November 2009, but for reasons unrelated to Silverlight.