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9.4. Fixed Positioning

As implied in the previous section, fixed positioning is just like absolute positioning, except the containing block of a fixed element is always the viewport. In this case, the element is totally removed from the document's flow and does not have a position relative to any part of the document.

This can be exploited in a number of interesting ways. First off, it's possible to create frame-style interfaces using fixed positioning. Consider Figure 9-24, which shows a very common layout scheme.

Figure 9-24

Figure 9-24. Emulating frames with fixed positioning

This could be done using the following styles:

DIV#header {position: fixed; top: 0; bottom: 80%; left: 20%; right: 0;
background: gray;}
DIV#sidebar {position: fixed; top: 0; bottom: 0; left: 0; right: 80%;
background: silver;}

This will fix the header and sidebar to the top and side of the viewport, where they will remain regardless of how the document is scrolled. The drawback here, though, is that the rest of the document will be overlapped by the fixed elements. Therefore, the rest of the content should probably be contained in its own DIV and employ the following:

DIV#main {position: absolute; top: 20%; bottom: 0; left: 20%; right: 0;
overflow: scroll; background: white;}

It would even be possible to create small gaps between the three positioned DIVs by adding some appropriate margins, demonstrated in Figure 9-25:

BODY {background: black; color: silver;}  /* colors for safety's sake */
DIV#header {position: fixed; top: 0; bottom: 80%; left: 20%; right: 0;
background: gray; margin-bottom: 2px; color: yellow;}
DIV#sidebar {position: fixed; top: 0; bottom: 0; left: 0; right: 80%;
background: silver; margin-right: 2px; color: maroon;}
DIV#main {position: absolute; top: 20%; bottom: 0; left: 20%; right: 0;
overflow: scroll; background: white; color: black;}
Figure 9-25

Figure 9-25. Separating the "frames"

Given such a case, a tiled image could be applied to the BODY background. This image would show through the

First, let's establish how the height of a line is determined. Aline's height (or the height of the linebox) isdetermined by the height of its constituent elements and othercontent (such as text). It's important to understand thatline-height really only applies to inline elementsand other inline content and not to block-level elements. We can seta line-height value for a block-level element, butthe only way this will have any visual impact is by being applied to gaps created by the margins, which could certainly be widened if the author saw fit. For that matter, if a background image was of little importance, simple borders could be applied to the DIVs instead of margins.



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Figure 7-68

Figure 7-68. Floating images and element backgrounds

There is nothing different about the second example, except for the visible background. As you can see, the floated image sticks out of the bottom of its parent element. Of course, it did so in the first example, but it was less obvious there because we couldn't see the background then. There is nothing forbidden about this behavior.

WARNING

In practice, some browsers may not do this correctly. Instead, they