"em" and "strong" are locale and medium neutral ways to signal intent. For example, most non-Latin scripts don't have an italic form, but rather use a different mechanism to signal emphasis. As well, screen readers and even high-contrast views may not work well with such things as italic and bold, and need a different mechanism to signal emphasis and strong.
Also, underline doesn't work for vertically oriented scripts, and is usually just another way to emphasize, which is why it was moved to styling instead.
Also, underline doesn't work for vertically oriented scripts, and is usually just another way to emphasize, which is why it was moved to styling instead.