The outlet reported that Kate felt the celebration was important to be at if she was feeling up to it.
Markle is another royal who's found her go-to wares.
Anyone meeting Meghan Markle for the first time might easily be star-struck - a well-known actress turned duchess isn't someone you come across very often.
Ms Seward told People.com: "She has blossomed and looked more confident, beaming with happiness".
She's expected to make her official debut back into the workforce in September, however.
She teamed the look with Cartier diamond and white gold earrings, which are a favourite of hers.
Now at the christening of Prince Louis Middleton high hairstyle appeared in a bright dress from Alexander McQueen.
She wore one to Princess Charlotte's christening in July 2015 and repeated the look for the Trooping of the Color in July 2016 and a commemoration ceremony in Belgium in 2017.
However, for the Air Force event, Kate accessorised with a matching headpiece by Sean Barrett featuring netting and flower detail as well as nude pointed-toe pumps.
Charles, Camilla, William, Kate, Harry and Meghan will also meet veterans, personnel, charity representatives and those from the civil service in a reception after the spectacle.
Royal experts have continued to draw comparisons between the two women, ever since Kate and Prince William announced their engagement in 2010.
Led by the Queen, the royals were attending a service to mark the centenary of the Royal Air Force.
Kate Middleton wore a light brown hat designed by Jane Taylor to Prince Louis's christening.
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