Following accusations that they are trying to force the world into a proprietary form of web development through Flash, Adobe reacted by attempting to build a standard compliant ActionScript version. In this regard, they created ActionScript 3.0 according to a draft version of ECMAScript called ECMAScript 4.0 (ES4).
What followed is what many consider a public political war between Microsoft and Adobe. While ActionScript 3.0 registered a huge success and is widely appreciated by developers, Microsoft made it quite clear that it would not support ES4, the official reason being that ES4 brings too many changes and has features that are more suitable for compilation-based environments, but not for the whole wide web. The ECMA Technical Committee, that incorporates representatives from Adobe, Mozilla, Microsoft, Apple, Opera, Google, Yahoo and other big companies, was split in half for over a year and some committee members publicly countered each other through blog articles.
Things are not too harmonious for Adobe though, as this means that the plans of ActionScript 3 being standard compliant are now over. How this will reflect ActionScript's future is not yet decided. Dan Smith, Adobe community expert, thinks that ActionScript will conform to ECMAScript 3.1 Harmony, but that it will keep the already implemented features of ES4 that have been scrapped for ES3.1. Other users feel that ActionScript will become an independent programming language because it's driven by the market rather than the standards. This means that if developers adopt it, and a lot have already done so, it doesn't really matter if it is compliant to any standard.
Some opinions point out that Microsoft could be the victim of its own decision, in terms of Internet Explorer market share and image. The reasoning behind this is that developers now point at Microsoft as responsible for trying to kill AS3 and encourage Mozilla to implement the dropped ES4 features, which would give Firefox a serious edge over Internet Explorer. Other more eccentric developers think Adobe should release a less feature-rich Flash player plug-in for Internet Explorer or even drop support for it entirely; however, this is unlikely to happen. One thing remains true though: for the time being there are a lot more Flash developers than Silverlight ones.
Adobe's Standardization Plans for RIA Development Platform Get Crushed
ECMA's Technical Committee ditches draft version of ECMAScript 4.0
... so hot right now