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Re: Where exactly is the "principal block box"?

From: Bert Bos <Bert.Bos@sophia.inria.fr>
Date: Wed, 12 Aug 1998 16:07:38 +0200 (MET DST)
To: <braden@endoframe.com>
Cc: "W3C Style Sheets Mailing List (E-mail)" <www-style@w3.org>
Message-ID: <13777.39912.715191.7986@www43.inria.fr>
Braden N. McDaniel writes:
 > The description of list-style-position says:
 > 
 > "This property specifies the position of the marker box in the principal
 > block box."
 > 
 > Referring to section 9.2.1:
 > 
 > "Block-level elements generate a principal block box that only contains
 > block boxes. The principal block box establishes the containing block for
 > descendant boxes and generated content and is also the box involved in any
 > positioning scheme."
 > 
 > I'm trying to establish where the boundary of the principal block box should
 > be. My interpretation from this is that it is the outer extremity of the
 > margin. Is that accurate?

The "principal box" is not defined as a rectangle. It refers to a box
generated by a block object as a whole, not to any of that box's edges
in particular.

 > 
 > The description of list-style-position goes on to describe the behavior of
 > the "outside" value:
 > 
 > "The marker box is outside the principal block box. ..."
 > 
 > Does this mean that "outside" specifies that the marker should be outside
 > the margin of the list item?

Not necessarily.

There is more info in 12.6.1 "Markers: the 'marker-offset' property":

  "The 'marker-offset' property specifies the horizontal offset between
  a marker box and the associated principal box. The distance is
  measured between their nearest border edges."

So, "outside" should be interpreted as "outside the border."


                              +-----------------------------M-------+
    +-----B--------+          | +---------------------------B-----+ |
    | +---P------+ |  marker- | | +-------------------------P---+ | |
    | | +-C----+ | |  offset  | | | +-----------------------C-+ | | |
    | | |marker| | |<==========>| | |principal box            | | | |
    | | +------+ | |          | | | |                         | | | |
    | +----------+ |          | | | |                         | | | |
    +--------------+          | | | |                         | | | |
                              | | | |                         | | | |
                              | | | |                         | | | |
                              | | | |                         | | | |
    M = margin edge           | | | |                         | | | |
    B = border edge           | | | +-------------------------+ | | |
    P = padding edge          | | +-----------------------------+ | |
    C = content edge          | +---------------------------------+ |
                              +-------------------------------------+


If 'marker-offset' is 'auto' (the initial value), the UA is free to
choose a suitable offset.




Bert
-- 
  Bert Bos                                ( W 3 C ) http://www.w3.org/
  http://www.w3.org/people/bos/                              W3C/INRIA
  bert@w3.org                             2004 Rt des Lucioles / BP 93
  +33 (0)4 92 38 76 92            06902 Sophia Antipolis Cedex, France
Received on Wednesday, 12 August 1998 10:08:13 GMT

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