[pcwalton]

We're talking about Web browsers. Technologies that try to integrate into Web browsers specifically but don't have a good compatibility story with the platform haven't had a lot of success.

[mambodog]

Some examples of non-standard extensions which have fallen by the wayside (or failed outright):

    ActiveX
    Java Applets
    Flash
    Silverlight
    VBScript (and other non-JS scripting langs)
    NaCL
    Dart (via a native VM)