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CNN  — 

With a career spanning three decades, British fashion critic Colin McDowell knows a thing or two about aristocratic dress codes – so much, in fact, that he wrote a book, “A Hundred Years of Royal Style,” on the subject.

So it should come as no surprise that he’s been closely following the developments surrounding Prince Harry and Meghan Markle’s wedding.

“I really am interested in this (wedding) because it’s breaking all the rules,” said McDowell, who expects the ceremony to take on a decidedly informal tone – but not too informal. Royal events are, after all, bound to certain dress codes and traditions.

“It’s all about the appearance,” he said. “Going back in history, it was only the aristocracy who were invited and they knew the code.”

A rainbow of hats worn by guests at the wedding of Prince William and Kate Middleton in 2011.

Guests, he says, will be “dressing down in a very correct way.” While the ebb and flow of fashion trends make it hard to pinpoint exactly which trends will prevail, McDowell is certain women will wear hats and high heels, in keeping with tradition, and predicts that a few men will opt to wear sneakers over dress shoes.

For all the guests attending this year’s royal wedding, he suggests one guiding principle: “Simplicity is the rule.”

Watch the video above to learn more about the royal wedding dress code.