That was the finding from Web site monitoring company Gomez, whose CTO, Imad Mouline, said it's critical for large retailers to test their online stores early and thoroughly in preparation for the holidays, when these companies do heavy business.

Since consumers are expected to spend less than originally forecast, due to the crumbling economy, online retailers need every shopper they can get and can't afford to lose them to technical problems. "There is more pressure this year than in previous ones because of the economy," Mouline said. "The implications are greater than in years past."

Running an online store that can withstand spikes in traffic is getting tougher because these Web sites are getting more complicated, he said. For example, in the past 12 months, online stores have featured significantly more rich media content, such as photos and videos, as well as more interactive capabilities, he said.

In addition, these stores, more than ever, are made up of multiple components powered by an increasing number of external providers. These can be outsourcers that handle a specific task or partners that interface with the site via mashups, Mouline said.

Adding to the complexity is a new generation of browsers that either have recently been shipped or will be shipped soon from vendors including Microsoft, Mozilla, Google and Apple. The new browsers are significantly different from their predecessors, and online stores need to be adapted for them.