"Strong" and "emphasis" are used for accessibility purposes.
For example, person blind since birth won't know what "italic" looks like. But they will understand the concept of emphasizing something.
And before you reply to me: I'm talking about compliance standards designed for use throughout the Internet. I'm not just sharing my opinion on this. My opinions are irrelevant. When I work in UX, I follow the standards.
No. They are tested standards, and were created based on extensive data and research.