One of the most frequent 2.1 issues I encounter is a form field border that doesn't have enough contrast with its background. It would be great if this were converted into a guided automatic.
My understanding of the SC is that a border passes if one of the following three conditions is met:
The border has at least 3:1 contrast with the background (hopefully the same algorithm as is used with text could calculate the background color)
The border has at least 3:1 contrast with the interior background color of the field
The background and the interior color of the field have at least 3:1 contrast with each other, regardless of the field color. (Understanding Success Criterion 1.4.11 refers to this as "subsuming into the background").