When Ruba Zai uploaded her first video online, the Netherlands-based Afghan student just wanted to share with other Muslim girls and women how she styled her headscarf. She had no idea her "hijab tutorials" would be an internet hit, watched by hundreds of thousands worldwide.

The 23-year-old now blogs full time, sharing ideas for how to look fashionable yet covered-up with a million Instagram followers. Zai had tapped into a fast-growing market for so-called "modest fashion," fueled by young, style-savvy Muslim women from London to Malaysia who have long felt their needs ignored by mainstream designers.

"I just couldn't relate at all to the clothes you see in the mainstream brands," Zai said from her home in Rotterdam. "When we first started talking about our style on social media, there was no interest in the fashion world in this group of people: 'They're just Muslims, why should we target them?'"

Big brands are waking up to that call, and covered-up chic is a niche that's slowly making its way into mainstream fashion. From exclusive designers to fast-fashion chains, retailers are trying to court millions of Muslim consumers - especially around the month of Ramadan, which started last week, when many Muslims buy new clothes and dress up. In 2014, U.S. fashion house DKNY was one of the first Western brands to launch a Ramadan collection aimed at wealthy Arab shoppers in the Gulf.

Since then, several others have followed suit. Dolce&Gabbana has been selling a luxury collection of abayas - long, loose robe-like dresses - and matching headscarves since last year in the Middle East, Paris and London. At the more affordable end of the market, Spanish chain Mango is also promoting a Ramadan collection of tunics, kaftans and maxi dresses for the second year.

Earlier this year, Nike became the first major brand to launch a "pro hijab," a headscarf made in high-tech fabrics aimed at female Muslim athletes. Even Marks and Spencer, that stalwart British department store known for cardigans and practical shoes, launched a $64 burkini - a full-body swimsuit - last summer in the U.K.

But perhaps the most visible sign yet that mainstream fashion is embracing the Muslim market was when design houses Max Mara and Alberta Ferretti starred hijab-wearing Somali-American model Halima Aden on their catwalks for Milan Fashion Week, one of the industry's most prestigious events.

"Mainstream fashion is now talking about modest fashion as a thing. Ten years ago, if you were a brand coming from a religious background and tried to sell it in department stores, calling it a modest or Muslim brand would be a kiss of death," said Reina Lewis, a professor at the London College of Fashion who has written two books about the topic.

While the majority of those interested in covered-up fashion are young, cosmopolitan Muslim women, "the term 'modesty' emerged in the niche market as a useful one because it's not faith-specific," Lewis added.

"I know Christians and atheist friends who don't cover their heads but they dress this (modest) way because that's how they feel most comfortable," said Zai.

Nazmin Alim, a designer who founded the London-based modest fashion brand Aab a decade ago, says she used to have to buy fabric herself and visit a tailor to get work wear that still adhered to her faith's modesty edicts.

"Long skirts may have a slit, tops may be sleeveless," she said. "We understood then that, do you know what? The people who wanted this kind of clothing, they are hungry for it."

This month, Alim's collection of trendy jumpsuits, kimonos and knee-length hoodies - as well as more traditional abayas and headscarves - is being sold at Debenhams, a British department store that says it's the first of its competitors to add hijabs to its aisles.

The fashion industry's attempts at carving a corner of this market haven't been without criticism, especially in France, where the banning of headscarves and burkinis amid racial tensions and security fears have fueled a heated debate.

Laurence Rossignol, the former French minister for women's affairs, was reported saying last year that major brands that promote Islamic dress were "irresponsible" and that such garments "promote the confinement of women's bodies."

Zai and Alim, however, felt strongly that for women like them, it's all about respecting individual choice.

"We all make choices - some people like to wear gothic, some people like what we're offering," Alim said. "I don't see why anyone's style should be singled out."

"I try to stay away from the political debate," said Zai, who said she decided to cover her head three years ago after a period of religious reflection. "I don't think a group of men - the people you see (in government) are all these old men - can tell people what's allowed, what's not allowed . they're saying Muslim women are oppressed, but they're doing the same."