The gooey-eyed, loved-up, you-can-do-no-wrong stage of the honeymoon between the new Duchess of Sussex and the media lasted for three weeks precisely. On Saturday 9 June, for her first appearance on the Buckingham Palace balcony after the trooping the colour celebrations, the royal formerly known as Meghan Markle wore a pale-pink Carolina Herrera dress with a wide, off-the-shoulder collar. “Some were quick to notice that she may have bent one of British royalty’s most steadfast rules,” observed Hello! magazine gravely, noting that “royal protocol usually advises that women should keep their shoulders covered”.

A few weeks later, at Wimbledon’s women’s singles final, the big news from the royal box was the daring return of the duchess’s signature “messy bun”. “Meghan Markle has been sticking to the royals’ elegant style lately … but this weekend her messy hairstyle and casual look were back,” noted Vanity Fair. As if strands of untucked hair were not enough to see the duchess branded the biggest royal rule-breaker since the Duchess of Devonshire scandalised 18th-century society with her affairs and gambling debts, it transpired that at one event she had momentarily forgotten the “duchess slant” and crossed her legs at the knee, rather than the ankle. The Daily Mail expressed concern that she would be “slammed for disrespecting the Queen”, quoting a “royal etiquette expert” who pronounced crossing the leg at the knee one of the “biggest etiquette mistakes a lady can make”.

Facebook Twitter Pinterest ‘They must do the heavy lifting on the global stage without looking as if they are upstaging the monarch’ ... the Duchesses of Cambridge and Sussex during Trooping the Colour. Photograph: Max Mumby/Indigo/Getty Images

It is 100 days since she became a duchess and the “breaking royal protocol” narrative has emerged as the plot twist that makes her fairytale a modern one. It is a tagline that has almost nothing to do with her relationship with the royal family and everything to do with her relationship with the public.

The first “scandal” of the duchess’s married life came when, at her father-in-law’s birthday party, she was photographed in sheer tights. It was reported that the tights were by order of the Queen, but I find it hard to believe the Queen is so petty as to issue hosiery edicts (the Duchess of Cambridge does not always abide by them, if so). It seems more likely that the tights were simply an attempt at looking formal that backfired in the face of flash photography. Nonetheless, little else seemed to be discussed on social media for days. To brandish a pair of unphotogenic tights as proof that a 36-year-old woman who had weathered a pre-wedding familial firestorm and maintained grace and dignity in front of a wedding audience of millions had surrendered free will and submitted to Gilead-esque rules within days seems to stretch the significance of hosiery.

The duchess’s image as a rule breaker is catnip because it cuts both ways. It energises those who disapprove of her and those who cheer her. Even when the breaking of royal protocol is used as admonishment, many among the public see it as her greatest asset. As the first biracial woman to marry into the royal family, the first to have had a successful career, the first self-proclaimed feminist, she is a revolutionary force in Britain just by being herself. Everything that happens in her orbit – from Bishop Michael Curry referencing Martin Luther King during her wedding ceremony to a few strands of hair escaping from her updo – is seen through this lens.

Facebook Twitter Pinterest ‘Those of us who do not want her in sheer tights would prefer to have her in ripped jeans and a white shirt’ ... with her then boyfriend at the 2017 Invictus Games in Toronto, Canada. Photograph: Danny Lawson/PA

The discussion of her clothes is never really about clothes. When Kate Middleton married Prince William seven years ago, the interest in her wardrobe settled on the price tags. The forensic examination of how much she had paid for her LK Bennett shoes and Reiss jackets was a thinly veiled vehicle for the sneery obsession with her having come from a family which was not quite posh enough. With the Duchess of Sussex, our interest is in how exciting and refreshing she is as a royal, so the focus on her wardrobe zeroes in on how formal or informal it is.

Facebook Twitter Pinterest Loved by fashion critics ... the Givenchy trousersuit she wore on an official visit to Dublin. Photograph: Getty Images

As a rule, the British public likes its royal women to be as glamorous as possible, but around the duchess there is a new appetite for deshabille. Her outfit for Wimbledon – the messy bun, along with a striped, blue-and-white Ralph Lauren shirt and cream palazzo pants – was her best-received outfit since the wedding. This was a more polished version of the oversized white shirt and ripped blue jeans she wore for her first public appearance with her then boyfriend in September last year. A sharp but understated black Givenchy trousersuit, worn on 11 July during an official visit to Dublin, was also loved by fashion critics. By contrast, the voluminous blue-and-white toile de jouy Oscar de la Renta maxi dress she wore to the wedding of her husband’s cousin Celia McCorquodale – by far her most daring, fashion-forward choice of the past 100 days – got a cold reception. No one wants fashion-forward from the duchess, it seems. Those of us who do not want her in sheer tights would prefer to have her in ripped jeans and a white shirt.

She has developed a signature look that appeals to as broad a church as possible. The most distinctive feature is the wide, bateau neckline that featured on her Givenchy wedding dress and in which she has been seen many times since. This neckline has featured on a caped, cream Givenchy dress, but also on a navy belted Dior number and an olive-green Ralph Lauren ensemble. This, therefore, is a directive coming from the wearer, not the designer. Tailored dresses such as this are favoured by women in the public eye all over the world because a sculpted shape is reliably flattering from every angle, but they can be a little ice-queen in tone.

The bateau neckline is a grownup, non-tawdry way to show enough skin to lend warmth to this look. It is a preppy, non-specifically vintage style favoured by a certain kind of upscale, Europeanised American woman – Grace Kelly, Carolyn Bessette-Kennedy and now the Duchess of Sussex.

Play Video 1:14 Meghan Markle's wedding dress: the first glimpse - video

The interest in the wardrobes of the Duchesses of Sussex and Cambridge is intensified by the generational balance of the royal family. That the Queen is a bona fide style icon was proved beyond doubt even before she wore a brooch given to her by Barack and Michelle Obama to a meeting with Donald Trump, but she is also 92 and unlikely to experiment with new silhouettes or hairstyles. In the royal glamour stakes, the duchesses must do the heavy lifting on the global stage without looking as if they are upstaging the monarch. The predominance of bridesmaid-style blush pink and demure navy in their wardrobes – one sunshine-yellow sheath dress from the Duchess of Sussex being a notable exception – is a show of deference to the Queen owning bright colours at public events.

Our new royal is compelling to watch because the balancing act she must pull off is almost impossible. The Duchess of Sussex is expected to show us the way towards the royal family of the future, but without showing disrespect for the royal family of today. We cheer her on for breaking royal protocol, but we admire and praise her when she remains aloof of the scandals that swirl around her. We want our royal women to be real and human and modern these days – and that is progress. The catch is that we still want them to be perfect.

• This article was amended on 23 August 2018 because an earlier version erroneously referred to the Duchess of Sussex as the first divorcee to marry into the royal family.

