When I first starting writing my previous post, I had envisioned a larger comparison between white wading birds. I abandoned that idea once I realized the number of white waders for which I had no photograph: Cattle Egret, immature Little Blue Heron, white morph of Reddish Egret. And then I realized that I was ignoring several other birds, such as White Ibis, Wood Stork, immature Roseate Spoonbill and even Whooping Crane. This quickly overwhelmed my poor brain, and thus my previous post turned into the much simpler comparison of just two white waders.
I still, however, had this photograph of a Snowy Egret, also taken from the grounds of Florida Keys Wild Bird Center. There was a viewing area at the back of the grounds, and this Snowy Egret was perched in clear view, intent on some personal grooming.
The obvious field mark is the yellow feet. The black bill and legs are also important, especially when the feet are under water. More subtle marks are the yellow loral area (between the eye and bill), and the yellow lines down the back of the legs (barely visible in this photograph).
[edit: added missing word in last sentence of first paragraph]