CODEWITHSUNDEEP
css-floatmargincss
Lee
Lee

Reputation: 1641

Why is the left margin jumping up?

I'm following In Search of the Holy Grail, but am having trouble understanding what's actually going on in step 3.

To break the problem down, I disabled the margins/offsets of the "left" and "right" divs. This gives you the content at the top, and then the two other elements sitting side by side under it as I would expect [see P1].

I then gradually decreased the margin-left of the "left" div from 0px to -199px which again does what I expect [see P2].

But at -200px (the width of the left element itself), it shoots up to the top [see P3], with the left edge against the right edge of the content. I would have expected it to just keep going off the edge.

Why does it jump up? And if there's no conceptual explanation as to why, where does it describe that functionality in the spec?

Images:

P1

Photo 1

P2

Photo 2

P3

Photo 3

Upvotes: 2

Views: 533

Answers (2)

BoltClock
BoltClock

Reputation: 723528

Remember that both #content and #left (as well as #right) are floated.

Due to the nature of floats, they (or their contents) may overlap. This is well described in this section of the spec, and is fairly easy to understand. If you apply negative margins to a floated element that's adjacent to another float, it will simply move to its left (similar to having a left relative offset), over its sibling.

In the section on the float property, you'll find a list of "the precise rules that govern the behavior of floats." Now, I'm not 100% familiar with the float model, but these points are what I believe to be relevant:

2. If the current box is left-floating, and there are any left-floating boxes generated by elements earlier in the source document, then for each such earlier box, either the left outer edge of the current box must be to the right of the right outer edge of the earlier box, or its top must be lower than the bottom of the earlier box. Analogous rules hold for right-floating boxes.1

7. A left-floating box that has another left-floating box to its left may not have its right outer edge to the right of its containing block's right edge. (Loosely: a left float may not stick out at the right edge, unless it is already as far to the left as possible.) An analogous rule holds for right-floating elements.

8. A floating box must be placed as high as possible.

9. A left-floating box must be put as far to the left as possible, a right-floating box as far to the right as possible. A higher position is preferred over one that is further to the left/right.

So my guess is: a margin of -200px, which is as you say the negative equivalent of the width of the #left element itself, causes it to float all the way up and to the right (rather than to the left) and to hug the edge of the #center element which itself is also floated. The right edge of both elements touch each other because of this full, equal negative margin. Consequently, as you continue to increase (or decrease?) the negative margin, you'll see that the #left element continues to move to its left.

Note that the padding applied to the #container element doesn't interact with the margins; even if you remove the padding on either side or both sides, the margins will interact in the same way.

1 Note also that there's a statement in the section on collapsing margins in the linked section of the spec, that describes the behavior of negative margins, but that is irrelevant as the margins we are concerned with here are horizontal and belong to floated elements, and so are never affected by collapsing.

Upvotes: 2

CSS Guy
CSS Guy

Reputation: 1984

you are using <h2> tag for left and right heading but in center you are using <h1> that's why you have this problem if you want to solve this problem do one thing 1 use all <h2> tag for heading and if you want to use then apply below class on

h1
{
margin-top:10px;
}

Upvotes: 0

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