I did a test on html5test.com on the new Playbook OS webbrowser abilities... It ranks highest among the tablets at 393 (Chrome is runner up at 371)
However beside runnerup they have upcoming and Blackberry 10 scored 465 out of 500 on the test, that is higher than the fastest desktop browsers as (current or upcoming), in fact it is higher than any other browser on any device (at least for now)
Check it out here
The HTML5 test - How well does your browser support HTML5?