Why do intelligent people who write for the web get this wrong?
principal:
When used as a noun it means the ‘boss’ the leader, the person in charge.
When used as an adjective it means the first, the foremost, the most important.
principle:
It is a noun. It is not an adjective. It means the rule of conduct, the laws that underly how a system works, the things I stand by.
Example:
“The principal stood by his principles of equality and invited everyone to the school party, including the cleaners. It was unheard of at that time, and we all knew we had a boss who deserved our loyalty.”