Book HomeCascading Style Sheets: The Definitive GuideSearch this book Wednesday 01st of October 2014 04:13:46 PM

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;
reason to do so. The following two rules mean exactly the same thing:

*.apple {color: red;}.apple {color: red;}

However, you should consider this: if you're concerned aboutolder user agents that don't know about CSS2, then*.class (or *#id) is an easyway to fool them. Since both of these are examples of invalidselectors in CSS1, they should be ignored by CSS1-only parsers. Ifthey aren't ignored, then they're likely to cause strange 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 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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B {padding-left: 10px; padding-right: 10px; background: silver;}
Figure 7-60

Figure 7-60. Padding on an inline element

Note the extra background space that appears on either end of theboldfaced text. There's your padding.

This all seems familiar enough, even when the boldfaced textstretches across multiple lines. Turn to Figure 7-61to see what happens with padding set on an inline element displayedacross multiple lines: