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While observers weren't necessarily expecting a bow or curtsy, they may not have predicted a greeting this informal, either.

U.S. President Donald Trump's historic second state visit to Britain began Wednesday with a stop at Windsor Castle, where he was met by Prince William and Catherine, Princess of Wales.

Trump stepped out of his helicopter, raised his hand in a salute and then shook William's hand. Then, upon greeting Catherine, Trump shook her hand and said,"You're beautiful. So beautiful."

Catherine didn't appear to react one way or another. She reached out to shake first lady Melania Trump's hand, and the foursome walked across the lawn toward King Charles III and Queen Camilla.

There are no obligatory codes of conduct when meeting a member of the Royal Family, notes the official royal website, adding that many people do still wish to"observe the traditional forms" — which for men, is a neck bow.

WATCH | The moment when Trump greeted Catherine:

Trump tells Catherine, Princess of Wales, she's 'so beautiful'

3 hours ago

So technically, Trump didn't breach royal protocol by commenting on Catherine's appearance, explained Justin Vovk, a royal historian with the Institute for the Study of the Crown in Canada.

"But it would, in my opinion, sort of transgress the unofficial rules of personal diplomacy," Vovk told CBC News.

Later Wednesday, at the formal state banquet, Trump mentioned Catherine's appearance again, this time in his official speech that was broadcast around the world.

"Melania and I are delighted to visit again with Prince William and to see Her Royal Highness Princess Catherine so radiant, and so healthy, and so beautiful," he said, reading solemnly off written notes, Catherine sitting to his right.

WATCH | Trump describes Catherine as 'so radiant':

Trump says Catherine 'so radiant' at state dinner

55 minutes ago

Trump has a history of making comments such as these in public, formal settings, likely thinking he comes off as charming or that he's breaking the ice, Vovk said.

But it's unprofessional, he added, particularly when meeting members of the Royal Family — who are used to getting comments about their appearance from the general public, but not one on one, and especially not during a formal state visit when the monarchy's role is to put its best foot forward.

"It was very awkward. There are so many levels of cringe to this," he said."How do you respond to something like that in a way that will not make the situation more awkward or will not come off as a rebuff of the compliment?"

A short history of Trump's physical comments

It's certainly not the first time Trump has made remarks, both good and bad, about a person's physical appearance. In fact, he's previously described Prince William as"a good-looking guy" and "very handsome."

These types of compliments, especially when made about women, have received mixed reactions. Just last month, he faced criticism for his, gushing about"that face" and "those lips, the way they move."

Also in August, he appeared to say,"You look fantastic" to Italian Prime Minister Giorgia Meloni during a meeting at the White House. In June, during the signing of a peace deal between the Democratic Republic of Congo and Rwanda, he called a female reporter in the Oval Office"beautiful" and added,"I wish I had more reporters like you."

Last October, he called Alabama Sen. Katie Britt"fantastically attractive" in a town hall.

It's also not Trump's first time making such a comment during a state visit. In 2017, on a state visit to France, he, saying"you're in such good shape," then turning to French President Emmanuel Macron and adding,"She's in such good physical shape."

'A bit odd'

So while Trump's comment to Catherine was "a bit odd," people have come to expect this sort of casualness with him, Robert Finch, the dominion chairman of the Monarchist League of Canada told CBC News.

"He does tend to compliment people on their looks and appearance when he meets them. So, the Princess of Wales would be no different in that regard," Finch said.

"You have to wonder what went through her head when she met him. I'd love to be a fly on the wall when she and William chat about their day."

Still, making these kinds of comments to members of the Royal Family is"just generally not done," Vovk said.

"It's just considered poor taste," he said."I think it really speaks to the different styles of leadership and the different values and the cultural differences between somebody like the [U.S.] president and a 1,000-year-old institution like the British monarchy."

WATCH | Will Trump follow royal protocol?

Will Trump follow royal etiquette?

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Previous moments from U.S. President Donald Trump’s visits to the U.K. went viral for breaking royal etiquette. Here’s a look at some of the traditions he's getting another chance to follow at this year’s state visit.