The entire purpose of an element like <b> is what it looks like. If we're being inclusive, then the entire purpose of an element like <b> is what it looks like, how it sounds, how it feels in Braille, and so on. Nothing more. It does not map to some abstract concept.
It should be defined as: When rendered on a visual display device supporting bold font, it makes the text bold. The specific behavior is not guaranteed and may vary based on the user-agent. For example, screen readers will pronounce the text with emphasis.
