On SunSpider: "The WebKit SunSpider tests exercise less than 10% of the API’s available from JavaScript and many of the tests loop through the same code thousands of times. This approach is not representative of real world scenarios and favors some JavaScript engine architectures over others."
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There's a decent Microsoft blog post explaining which aspects of browser performance the different benchmarks test:
http://blogs.msdn.com/b/ie/archive/2010/09/14/performance-wh...
On SunSpider: "The WebKit SunSpider tests exercise less than 10% of the API’s available from JavaScript and many of the tests loop through the same code thousands of times. This approach is not representative of real world scenarios and favors some JavaScript engine architectures over others."