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Negative margins

From: Sho Kuwamoto <skuwamoto@macromedia.com>
Date: Mon, 21 Jul 1997 14:46:07 -0700
Message-Id: <v03007808aff981485cff@[192.168.65.140]>
To: www-style@w3.org
I'm having trouble figuring out how negative margins
on block-level elements should work. According to
section 4.1 of the spec, the top and bottom margins
between two block-level elements should work as follows:

         XXXXXXXXXXXXXXX
         XX element A XX
         XXXXXXXXXXXXXXX
                          the space here is the max of
                      <-- the bottom margin of A and the
                          top margin of B.
         XXXXXXXXXXXXXXX
         XX element B XX
         XXXXXXXXXXXXXXX


This is fine for positive values, but it gets hard to know
what to do for negative values. Let's say that the bottom
margin of A is 50px, and that the top margin of B is -10px.
Here are some possibilities:

1.1) Follow the spec literally.
       50px > -10px, thus the space between is 50px.

1.2) Add the two.
       50px-10px = 40px, so the space between is 40px.

Solution (1.2) is attractive because it seems to capture the
web author's intentions. By specifying a space-before of
-10px, he or she is probably saying "I want 10 pixels less
space than I would otherwise get." Now, consider the
situation when both are negative. Say the space after A is
-10px, and that the space before B is -20.

2.1) Follow the spec literally.
 	-10px > -20px, thus the space between is -10px.

2.2) Instead of using max(a,b), -max(-a,-b).
	20px > 10px, thus the space between is -20px.

2.3) Add the two.
	(-10px) + (-20px) = -30px, thus the space between is -30px.

Of these, I like (2.2) the best. It goes along with the spirit
of using max(a,b) for positive values.

So, combining (1.2) and (2.2), the algorithm to compute the
space between block-level elements would be:

if (a >= 0 && b>= 0)
	space = max(a,b);

else if ( (a >= 0 && b < 0) || (a < 0 && b >= 0) )
	space = a+b;

else // a < 0 && b < 0
	space = -max(-a,-b);

Any thoughts?

-Sho
Received on Monday, 21 July 1997 17:36:25 GMT

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