Those moments often occur around the time that online shopping reaches about 20 percent of total national retail spending in a category, the research firm L2 has concluded after studying the evolution of e-commerce. Online clothing and accessory shopping’s share of retail hit 21 percent last year, according to estimates by Cowen and Company, a stock research firm.

“I do think this year is the year apparel e-commerce takes off,” said Cooper Smith, an analyst at L2.

Apparel has been something of an e-commerce laggard. In years gone by, buying clothing over the internet was only for the fearless, with most shoppers unwilling to take the risk that a dress or a pair of shoes would fit poorly or look terrible on them.

It took time, but shopping habits for clothing are shifting profoundly.

Amazon’s solution was to improve clothing selection, pour money into photography to give internet shoppers a better representation of garments and offer free returns on most apparel so customers could order untroubled by the thought of sending items back.