The Hottest Trend in E-Commerce? Clothes.

Apparel is the fastest-growing segment in e-commerce this year, thanks to new ways to display clothing online.

EMarketer predicts that the apparel and accessories category is expected to grow by 20 percent to $40.9 billion this year, up from $34.2 billion in 2011.

That beats the sale of books, music and videos, which have historically been considered the most dominant e-commerce categories — those are now expected to grow by only 18 percent to $20.4 billion this year.

The report published by the Internet market research firm today is the first time it has broken down its forecasts by category.

EMarketer attributed the gains in apparel to retailers’ improved methods for displaying products online that make purchasing clothes, in particular, less of a guessing game.

Some of the visual innovations are stunning.

For instance,’s MyHabit, a private sales clothing site, produces animated videos of models wearing the clothing for sale. The representations are so realistic, the fabric sways as the model struts and turns as if walking down a catwalk.

On the more whimsical side of innovations is eBay’s augmented reality app that allows users to “try on” a pair of sunglasses by layering them over a picture of their face.

But in addition to new display technology, there are also many more apparel sites for consumers to choose from. Over the last two years, the idea of private sales or flash sales — offering expensive, high-end fashion at steep discounts — has spread like wildfire.

Free shipping also plays a role since it removes some of the risk for consumers: If they don’t like a shirt or pair of pants, they don’t have to pay a fee for delivery back to the retailer.

Overall, the report is predicting another big year for e-commerce. It expects U.S. retail sales to grow 15.4 percent to $224.2 billion in 2012, which is higher than eMarketer’s previous forecast that called for e-commerce to grow 11.3 percent to $209.3 billion in 2012.

The biggest category in 2012 is expected to be computer and consumer electronics, which have a higher price point, and are expected to bring in a total of $48.6 billion.

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