I am text inside the box using <p> tags

I am text outside the box. I am just regular body text on the page. That's because I am not in any div tags. I am under the div tags for the box on this page.

I am text inside the box without <p> tags

Used an <hr /> with clear:both and width:50% to separate the examples.

Width value but no height value
Add height value 1px
No height or width values