The most important addition to this edition of the standard is an ECMA test suite comprising over 10,000 tests meant to verify how various browsers support the language. Any developer can perform the test at http://test262.ecmascript.org. Both Microsoft and Google contributed each with over 5,000 tests from IE Testing Center and Sputnik, respectively, and the list is open, the standards body promising to publish it as an Ecma Technical Report in December with later updates.
It is expected that major browsers will pass all tests or nearly all of them in the near future, except for Opera which needs to play some catch up.
It is noteworthy that all major browser companies are coming together to have mutually agreed HTML5 and ECMAScript standards, which is very needed for a unified web experience from the user’s perspective, the competition moving towards services provided and ecosystems created around them. That is a clear separation from the past, at least from this perspective, when Microsoft and Netscape fought bitterly to dominate the web experience by providing browsers that were incompatible with each other in many ways.