The online-only retailer grew sales by 11.8% in 2014.
The retailer led both Internet Explorer and Firefox availability tests.
Amway.com had the best availability score in Internet Explorer and Firefox testing among large retailers tested in September, says Compuware Gomez. Last month, the retailer posted 100% availability in tests for both browsers.
Williams-Sonoma.com finished second behind Amway.com in Internet Explorer tests with 99.99% availability. Gap.com (99.98%) was next, followed by Toysrus.com (99.89%) and CDW.com (99.88%).
In Firefox testing, Gap.com placed second behind Amway.com with a 99.99% availability score for the month. Williams-Sonoma.com (99.94%) placed third, and CDW.com (99.92%) and Dell.com (99.91%) rounded out the top five.
The average Internet Explorer availability score among the 20 retailers measured in September was 99.24%. The same group of retailers produced an average Firefox score of 99.41%.
Availability is determined by the percentage of tests that load correctly.
Click here to view the full test results.