Question

First off, this example will only work in a browser that supports :nth-child, like Chrome or FireFox. I have an unordered list where the odd numbered list items are floated left and clear left, and the even numbered list items float right and clear right. Like so:

HTML:

<ul class="waterfall">
    <li style="height: 100px;">1</li>
    <li style="height: 200px;">2</li>
    <li style="height: 50px;">3</li>
    <li style="height: 100px;">4</li>
    <li style="height: 200px;">5</li>
    <li style="height: 50px;">6</li>
    <li style="height: 50px;">7</li>
    <li style="height: 50px;">8</li>
</ul>​

CSS:

.waterfall {width:302px;}

.waterfall LI {
    min-width: 150px;
    background-color: #CCC;
    margin-top: 2px;
}
.waterfall LI:nth-child(odd) {
    float: left;
    clear: left;
    text-align: right;
}
.waterfall LI:nth-child(even) {
    float: right;
    clear: right;
    text-align: left;
}​

Here’s the fiddle.

Goal:

All odd numbered list items should be floated to the left and be stacked on top of one another with no gap in between except the 2px margin. The same is true with the even numbered list items, except they should be floated right.

Questions a nutshell:

I’m confused as to exactly why, in this example, LI #5 can’t appear above LI #4 and LI #8 can’t appear above LI #7. In other words, why is LI #5 clearing LI #2, and LI #8 clearing LI #5? Also, why doesn’t LI #3 clear LI #2 (I don’t want it to, but if LI #5 is clearing LI #2, why doesn’t LI #3 join the party)?

TL;DR

My thoughts so far are somewhat inconclusive. It appears that the browser doesn’t want the top of an element to appear above the top of another element if the first element is defined later in the markup. Is that true?

I understand that floated elements are placed on a line, and that any floated elements not fitting on that line will just jump down to a new line and flow from there (as opposed to the free-flow concept of absolutely positioned elements). In other words, a floated element is removed from the document flow, but not quite as literally as an absolutely positioned element, because the floated element is still positioned relative to whatever line it started from and it still takes up space in the document flow. So, how is the next “line” from which floated elements hang from determined? In this example, why isn’t LI #3 clearing LI #2, but LI #5 is?

What I’m NOT looking for:

A clever solution to make this layout work by changing the markup, abandoning floats, or using JavaScript. I know how to do all that. I simply wish to be schooled in the mysterious ways of the float.

Answer 1

From the CSS 2.1 spec:

Here are the precise rules that govern the behavior of floats:

1 The left outer edge of a left-floating box may not be to the left
of the left edge of its containing block. An analogous rule holds for
right-floating elements.

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.

3 The
right outer edge of a left-floating box may not be to the right of the
left outer edge of any right-floating box that is next to it.
Analogous rules hold for right-floating elements.

4 A floating
box’s outer top may not be higher than the top of its containing
block. When the float occurs between two collapsing margins, the float
is positioned as if it had an otherwise empty anonymous block parent
taking part in the flow. The position of such a parent is defined by
the rules in the section on margin collapsing.

5 The outer top of a
floating box may not be higher than the outer top of any block or
floated box generated by an element earlier in the source document.

6 The outer top of an element’s floating box may not be higher than
the top of any line-box containing a box generated by an element
earlier in the source document.

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

It appears that the browser doesn’t want the top of an element to
appear above the top of another element if the first element is
defined later in the markup. Is that true?

Yes. It’s covered by rules 5 and 6 above.