Right click on the "all" dropdown in the sentence "Match all of the fields below".
Select "Inspect Accessibility Properties"
Note that the name is "null", and that you get an alert "Form elements must be labeled". Expected behavior is that the form element does have an accessible name.
Repeat steps 2-4 on the "relevance" sort dropdown at the end of the form.
Suggested fixes
Add an aria-label to the first dropdown
Associate the "relevance" dropdown with the "Sort results by" label, perhaps using aria-labelledby.
Steps to recreate
In Firefox:
Suggested fixes