px
The reference pixel — the honest lie that props up all the others.
One px is not your monitor's pixel: it is 1/96 of an inch, defined by the standard as a reference pixel — the visual angle of one dot on a 96dpi screen seen at arm's length. Dense screens pack two or three physical dots inside each one, at the rate dppx measures.
It is the right choice for whatever talks to the rasterizer: hairline borders, one-pixel shadows, small radii — details that should not grow along with the user's font. For text and layout, the relative units measure better.
.apara {
border-top: 1px solid;
}