you're hilarious.
There was no rule that stated candidates had to be white, there was no rule candidates had to show tax returns.
The lack of knowledge of the posters here......
Wait, wait... do you not understand what a norm is? How do you, like... live?
When there is a common standard everybody understands, even if it's not physically written down, that's a social norm, a standard, a rule. Every candidate for the last 40 years released tax returns. Releasing tax returns was politically expected of every political candidate. To obnoxiously quote marriam-webster, this makes releasing tax returns "an accepted procedure, custom, or habit." Or to obnoxiously quote Odford, it was " A principle that operates within a particular sphere of knowledge, describing or prescribing what is possible or allowable."
And thank goodness it's returning. Transparency is good. So codifying it into law is good.