Good point. Are capital-letter lists common? Lowercase letters appear to be common in Terms & Conditions. If capital-letter lists are uncommon, perhaps CommonMark could only support lowercase lists. The web developer could always use CSS to make the list capitalised if neccessary.
I have a concern with the double space rule after the B.. Writers who are “just muddling through” and haven’t read the documentation might be confused when the B. turns into a list item. Without searching the documentation on how to apply the override, they might give up.