This standard is missing a set of parentheses in the denominator of the fraction in the example.
“For example, 2/3 + 5/4 = 8/12 + 15/12 = 23/12. (In general, a/b + c/d = (ad + bc)/bd.)”
Should instead be:
“For example, 2/3 + 5/4 = 8/12 + 15/12 = 23/12. (In general, a/b + c/d = (ad + bc)/(bd).)”
I was about to write a long reply to this and then I discovered that George Bergman has already done it for me. Short version: there is no universally accepted convention that would require the parentheses, but they would indeed remove ambiguity. In this case I think the context (well-known formula for adding fractions) removes the ambiguity anyway. See the end of George’s article for a discussion of PEMDAS and its interpretations.