According to this quirksmode article, http://www.quirksmode.org/css/display.html
A block has some whitespace above and below it and tolerates no HTML
elements next to it, except when ordered
Are the whitespace above or below stated in pixels or is it just ‘whitespace’?.
In the given context, “whitespace” is a gross misnomer. Whitespace in terms of text should never directly interfere with the layout of non-inline block boxes; what you should see between block boxes are margins (usually of said boxes), which are completely different.
Margins are indeed stated in pixels. In fact, they may be stated with any CSS length unit; see the
marginproperties in the spec. You don’t specify a pixel length for whitespace directly for elements that flow inline; that is usually controlled byfont-sizeinstead, but when working with block boxes that should be entirely irrelevant.