When J. Crew started opening Madewell stores in 2006, the company declared that its new brand would deliver “real, honest women’s clothes” for hip 20- and 30-somethings. “Not too trendy, not too girly, just cool clothes infused with a modern upbeat attitude,” it said in a release at the time, promoting the worn-in feel of the retailer’s T-shirts, jeans and chinos.

The idea worked better, perhaps, than the company expected.

Madewell has since become a shining retail star, one that filed for an initial public offering on Friday at the same time that the once-popular J. Crew brand struggles to regain relevance in an increasingly online environment. The fortunes of the chains have diverged so much that Madewell’s I.P.O. will help raise much-needed capital for its parent, J. Crew, which will remain private.

While Madewell is only about one-third the size of J. Crew, the brand has taken off in recent years thanks to a retail strategy better suited to today’s shopping landscape and a broader shift to more casual clothing in the United States. Madewell said in its filing that it believed a substantial portion of the population anchored its wardrobes to denim — and not just on weekends “but also increasingly for work and social occasions.”

“There’s a real blurring now between what people are wearing on the weekend and what they’re wearing for work, and that blurring is working really well for brands such as Madewell,” said Sarah Willersdorf, a partner at Boston Consulting Group who specializes in fashion and luxury. Jeans are a particular beneficiary of the “casualization” trend among men and women, which has also fueled growth in athleisure and streetwear, she said.