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RE: [CSS2.1-9.5] [CSS2.1-9.4.1] Block formatting context and Floats

From: Anton Prowse <prowse@moonhenge.net>
Date: Mon, 19 Jan 2009 23:15:32 +0100
Message-ID: <4974FB84.7090302@moonhenge.net>
To: www-style@w3.org

This correspondence was confusing; however, the issue which I believe 
was being raised is a valid concern and needs clarification or 
resolution.   Note that for convenience the relevant passages from the 
spec are quoted in their entirety at the bottom of this mail.


9.5 (Floats) states that the border box of an in-flow block formatting 
context B must not overlap(*) any float F in the same block formatting 
context P as the element itself, but does not refer to the margin box of 
B.  [Other floats, being out of flow, are excluded from this 
requirement; this is in keeping with the following sentence higher up in 
9.5 which does explicitly refer to the margin box: "a floated box is 
shifted to the left or right until its outer edge touches the containing 
block edge or the outer edge of another float".]

(*) if "overlap" should be taken to mean "overlap the margin box of F" 
then for what follows let m be the length the right margin width of F. 
Otherwise, if "overlap" should be taken to mean "overlap the border box 
of F", let m be zero.


On the other hand, 9.4.1 (Block formatting contexts) states that a box 
B's left outer edge touches the left edge of the containing block P, 
even in the presence of the float F, unless B establishes a new block 
formatting context (in which case B may become narrower due to the 
presence of F).

Then, irrespective of whether or not B becomes narrower, if the UA 
decides that B can sit next to F and if the left margin of B is less 
than the sum
	s = (left margin of F) + (border area width of F) + m
then B must be shifted rightwards away from the left edge of the 
containing block P.  The issue is that it is not clear by how much.

Should the the right edge of F be coincident with the left /border/ edge 
of B or the left /margin/ edge of B?
If the latter, does this hold equally when the left margin of B is equal 
to or greater than s?
And are we talking about the right /border/ edge of F (m = 0)
or the right /margin/ edge of F?
If the latter, is there any need to exclude floats from the paragraph in 
9.5 referred to above?

This issue is tracked for the current latest release of IE8 beta as 
https://connect.microsoft.com/IE/feedback/ViewFeedback.aspx?FeedbackID=399822
Resolving this issue before IE8 is finalized would be highly desirable.


 From CSS21:
Section 9.5 Floats:
http://www.w3.org/TR/CSS21/visuren.html#bfc-next-to-float
The border box of a table, a block-level replaced element, or an element 
in the normal flow that establishes a new block formatting context (such 
as an element with 'overflow' other than 'visible') must not overlap any 
floats in the same block formatting context as the element itself. If 
necessary, implementations should clear the said element by placing it 
below any preceding floats, but may place it adjacent to such floats if 
there is sufficient space. They may even make the border box of said 
element narrower than defined by section 10.3.3. CSS2 does not define 
when a UA may put said element next to the float or by how much said 
element may become narrower.

Section 9.4.1 Block formatting contexts:
http://www.w3.org/TR/CSS21/visuren.html#block-formatting
In a block formatting context, each box's left outer edge touches the 
left edge of the containing block (for right-to-left formatting, right 
edges touch). This is true even in the presence of floats (although a 
box's line boxes may shrink due to the floats), unless the box 
establishes a new block formatting context (in which case the box itself 
may become narrower due to the floats).


Cheers,
Anton Prowse
http://dev.moonhenge.net
Received on Monday, 19 January 2009 22:16:21 GMT

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