Let me quote this answer:
display: inline means that the element is displayed inline, inside the current block on the same line. Only when it's between two blocks does the element form an 'anonymous block', that however has the smallest possible width.
But if top border would be taken into account, it would make your div
vertically misaligned with the other elements on the same line, even though in your case there is only a single element on the line. However, top border is ignored, therefore it is "sticking out" of the body and you cannot see it.
As a "proof", try to modify your HTML code in the provided fiddle as:
<div style="line-height: 50px"><div class="content">test test test</div></div>
Then you'll be able to see the top border, as the height of parent element has enough space for it not to stick out.