Css Position:fixed Without Top Gives Unexpected Layout?

Given:

Fixed div

Non-fixed div

Non-fix

Solution 1:

You'll notice that the "fixed" div is actually at the top of the body, the position and size of which match those of the "nonfixed" div.

This is most certainly due to the top margins of the body and div#nonfixed collapsing. See http://www.w3.org/TR/CSS21/box.html#collapsing-margins

8.3.1 Collapsing margins

In CSS, the adjoining margins of two or more boxes (which might or might not be siblings) can combine to form a single margin. Margins that combine this way are said to collapse, and the resulting combined margin is called a collapsed margin.

(...)

Two margins are adjoining if and only if:

• both belong to in-flow block-level boxes that participate in the same block formatting context
• no line boxes, no clearance, no padding and no border separate them (Note that certain zero-height line boxes (see 9.4.2) are ignored for this purpose.)
• both belong to vertically-adjacent box edges, i.e. form one of the following pairs:
  • top margin of a box and top margin of its first in-flow
  • (...)

The topis relative to the containing block, which is apparently not bodybut html (the root element).

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