closed as primarily opinion-based by MichaelT, BЈовић, Kilian Foth, GlenH7, Martijn Pieters Jul 29 at 12:02
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Our current iteration is based on ES5-strict support in modern browsers and executes at full speed and with a sufficiently simple translation as to be negligible during debugging. We're not quite done with all the features of the new version but you can try it out experimentally at http://caja.appspot.com/?es5=true . We expect our modern version of Caja to see a lot greater adoption.
We will continue to support older browsers with our existing server-side translation version and existing sites can start using it today and start getting the modern version based on the version of the browser soon. For example, Google Sites uses Caja to sandbox HTML Box in this way.
For more information on how to embed content using Caja, see https://developers.google.com/caja