Better than that, XS today achieves over 99% compatibility on test262, the official JavaScript language test suite, for ECMAScript 2017 (the latest versin of the JavaScript language). That's over 50,000 tests. Details on the test results in our GitHub repository @ https://github.com/Moddable-OpenSource/moddable/blob/public/...
[1] https://kangax.github.io/compat-table/es6/#xs6