Hybrids are barely touched upon in that tutorial, but here in the Pacific Northwest it is a major consideration. The most confounding hybrid we have is 'Cook Inlet' Gull (a Herring x Glaucous-winged Gull), which can look quite similar to both Thayer's and Herring Gulls. I would be interested in seeing the apparent Herring Gull with a pink orbital ring from the tutorial in full. Here's an example of a 'Cook Inlet' Gull that looks very much like a Herring Gull, but it has a pink orbital ring, the webs of the primaries are not truly black, the eyes are a little clouded, and the head has a certain smudgy appearance.
Cook Inlet Gull (Larus glaucescens x smithsonianus)
by Jeremy Gatten
, on Flickr
I don't mean to throw a wrench in the tutorial, but this is a reality of living in the Pacific Northwest. Learning gulls here is very hard, but also very rewarding. Often, you just have to throw up your hands and walk away because it is just a little too complex. I think learning how to document gulls is a good step in the right direction, and then getting some assistance with identification afterwards. Eventually you'll learn the classic individuals and the slightly atypical individuals... and then you just have to work extra hard to get comfortable with all the ages and hybrids. I personally enjoy it a lot!