Earlier quoted context omitted.
Javascript has a standard (or more accurately, current javascript is the implementation of a version of the standard). It's a much safer bet that either there will be backwards compatible implementations later or that you will have other options. To my knowlesde, IE's feature set was always "what's supported by the current release". It's sort of like having a codebase which you can't change which makes use of a bunch…
Even back in 1997 JavaScript worked differently based on what OS and web browser you had. I had open sourced this script to detect the OS and web browser so that it would run JavaScript code using if statements for each platform to solve compatibility problems. It should still work today but the ActiveX detection is broken because of the way JavaScript changed. http://www.javascriptsource.com/repository/javascripts/2…
It took quite a while for some standard JS features to be added[1], but a mostly stable subset was possible (as evidenced by the sites that worked in all the major browsers of the time). The real problem was companies not wanting to devote the time to doing this and taking the shortcut of using the non-standard IE features MS had included at the expense of browser interoperability (which would have provided some protection against using features that would be dropped).
1: https://msdn.microsoft.com/en-us/library/s4esdbwz(v=vs.94).a...