I have Spinal Muscular Atrophy and due to weakness can only use a mouse + on-screen keyboard.
My biggest gripe is sites that load with focus on an element that doesn't allow me to scroll down via the down arrow. This forces me to click the middle of the page to continue navigation via OSK.
Bad: https://circleci.com/gh/wbond/asn1crypto (can't even page down) https://pypi.python.org/pypi (new pypi might change this)