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4.2. Positioning Attributes

The CSS-Positioning recommendation specifies several properties that can be set as style sheet rule attributes. These attributes are used only when the position attribute is included in the rule; otherwise they have no meaning. Implementation of all the CSS-P attributes varies from browser to browser. Table 4.1 provides a summary of all the attributes defined in the W3C recommendation as well as how those attributes are implemented in the browsers. A separate column shows the Navigator <LAYER> tag attribute that corresponds to the CSS-P attribute.

Table 4.1. Summary of Positioning Attributes
CSS Attribute Description CSS-P IE NN Layer Attribute


Defines a style rule as being for a positionable element 1 4 4


The offset distance from the left edge of the element's positioning context to the left edge of the element's box 1 4 4


The offset distance from the top edge of the element's positioning context to the top edge of the element's box 1 4 4


The width of an absolute-positioned element's content 1 4 4


The height of an absolute-positioned element's content 1 4 4


The shape and dimension of the viewable area of an absolute-positioned element 1 4 4


How to handle content that exceeds its height/width settings 1 4 4


Whether a positionable element is visible or not 1 4 4


The stacking order of a positionable element 1 4 4

The implementation of these positioning attributes is not completely identical in both Version 4 browsers, but there is a large degree of compatibility, with the exception of the clip and overflow attributes.

4.2.1. left, top, height, and width Attributes

Four attributes deal with lengths, whether they are for positioning of the element or determining its physical dimensions on the page. Recall from Chapter 3 (Figure 3.2) that height and width refer to the size of the content, exclusive of any padding, borders, or margins assigned to the element. The left and top values, however, apply to the location of the box edges (content + padding + border + margin). When using the CSS syntax, each of these four attributes can be specified as a fixed length or a percentage. Fixed-length units are borrowed from the CSS specification, as shown in Table 4.2. Percentage values are specified with an optional + or - symbol, a number, and a % symbol. Percentage values are applied to the parent element's value.

Table 4.2. Length Value Units (CSS and CSS-P)
Length Unit Example Description


Element's font height


Element's font x-height


Pixel (precise length is depends on the display device)


Inch (absolute measure)


Centimeter (absolute measure)


Millimeter (absolute measure)


Point (equal to 1/72 of an inch)


Pica (equivalent to 12 points)

The length unit you choose should be based on the primary output device for the document. Most HTML pages are designed for output solely on a video display, so the pixel unit is most commonly used for length measures. But if you intend your output to be printed, you may obtain more accurate placement and relative alignment of elements if you use one of the absolute units: inch, centimeter, millimeter, point, or pica. Print quality also depends on the quality of the printing engine built into the browser.

For attributes of the <LAYER> tag that correspond to the CSS attributes, the values you assign do not include units. All measurements are in pixels.

Navigator 4 and Internet Explorer 4 also disagree on how to render certain types of block elements, as described at the end of Chapter 3. Navigator closes up the height of a block around its content, regardless of the height setting of the element. Moreover, any content, such as text, an image, or even a solid background color, is inset from the edges of the element by a forced padding of about three pixels that cannot be removed. On the other hand, if you define a positionable object via the <LAYER> tag in Navigator, these problems disappear, and the width and height attributes truly set the size of the block element.

4.2.2. The clip Attribute

A clipping region is a geometric area (currently limited to rectangles) through which you can see a positioned element's content. For example, if you include an image in a document, but want only a small rectangular segment of the whole image to appear, you can set the clip attribute of the element to limit the viewable area of the image to that smaller rectangle. It is important to remember that the element does not shrink in overall size for the purposes of document flow, but any area that is beyond the clipping rectangle becomes transparent, allowing elements below it in the stacking to show through. If you want to position the viewable, clipped region so that it appears without a transparent border, you must position the entire element (whose top left corner still governs the element's position in the grid). Similarly, because the clipping region encompasses viewable items such as borders, you must nest a clipped image inside another element that sets its own border.

Figure 4.6 demonstrates (in three stages) the concept of a clipping region relative to an image. It also shows how positioning a clipped view requires setting the location of the element based on the element's original size.

Figure 4.6. How element clipping works

Setting the values for a clip region requires slightly different thinking from how you might otherwise describe the points of an rectangle. The clip attribute includes a shape and four numeric values in the sequence of top, right, bottom, left—the same clockwise sequence used by CSS to assign values to edge-related attributes (borders, padding, and margins) of block-level elements. Moreover, the values are entered as a space-delimited sequence of values in this format:

clip:rect(top right bottom left)

In Figure 4.6, the goal is to crop out the critter from the image and align the clipped image to the left. The original image (396 by 84 pixels) is at the top. To trim the critter requires bringing in the left clip edge 98 pixels. The bottom, one-pixel rule is also clipped:

<SPAN STYLE="position:absolute; clip:rect(0px 396px 83px 98px)">
<IMG SRC="oraImage.gif" HEIGHT=84 WIDTH=396>

Then, to reposition this image so that the clipped area abuts the left edge of its positioning context, the style rule for the element must assign a negative value to take up the slack of the now blank space:

<SPAN STYLE="position:absolute; left:-98px; clip:rect(0px 396px 83px 98px)">
<IMG SRC="oraImage.gif" HEIGHT=84 WIDTH=396>


4.2.3. The overflow Attribute

If you define a fixed width and height for a relative- or absolute-positioned element, you can tell the browser how to handle content that extends beyond the physical dimensions of the element block. While the overflow attribute is defined to help in this regard, unfortunately the implementation of this attribute is not the same in Navigator 4 and Internet Explorer 4. Consider the following document fragment that affects how much of the upper left corner of an image appears in the browser window:

<SPAN STYLE="position:relative; width:50; height:50">
<IMG SRC="myImage.gif" HEIGHT=90 WIDTH=120>

In the previous example, even though the width and height style attributes are set for a SPAN wrapper around an image, the natural width and height of the image force both browsers to show every pixel of the image. In other words, the content overflows the edges of the block containing the image. By adding an overflow attribute and value to the style rule, you can instruct the browser to cut the view at the edges of the block defined by the style rule:

<SPAN STYLE="position:relative; width:50; height:50; overflow:hidden">
<IMG SRC="myImage.gif" HEIGHT=90 WIDTH=120>

Thus, any content (between the <SPAN> and </SPAN> tag pair) is clipped to the size of the SPAN element's box. Navigator 4, however, exhibits slightly different behavior in that the horizontal dimension is never clipped by the overflow attribute. In the preceding example, the visible portion of the image is 50 pixels square in Internet Explorer 4 and 120 pixels wide by 50 pixels high in Navigator 4. If you truly want to clip the view of any content, it is best to use the clip attribute (described in the previous section) to set the viewing boundaries of content.

Internet Explorer also supports an optional CSS-P recommendation for setting the overflow attribute to scroll . This setting automatically displays scrollbars (a full set, unfortunately) inside the clipped rectangle defined by the positioned element's height and width attributes. Content is clipped to the remaining visible space; the user clicks or drags the scrollbars to maneuver through the content (image or text). This attribute setting is not available in Navigator 4.

4.2.4. The visibility Attribute

The purpose of the visibility attribute is obvious: it makes an element visible or hidden. Unless the element is under script control, however, it is unlikely that you would bother setting the attribute's value (to inherit, visible, or hidden). There is rarely a need to load a normally visible HTML element into a page as hidden, unless you also have a script that changes its state as the user visits the page—perhaps in response to mouse clicks or a timed event.

It is, however, important to understand the difference between setting a positionable element's visibility attribute and setting the CSS display attribute to none. When a positionable element is set to be hidden, the space occupied by the element—whether it be a position in the stacking order or the location for flowed content set off as a relative-positioned element—does not go away. If you hide a relative-positioned element that happens to be an emphasized chunk of text within a sentence, the rest of the sentence text does not close up when the positioned portion is hidden.

In contrast, if you set the CSS attribute of an element to display:none, this tells the browser to ignore the element as it flows the document. Navigator 4 does not have a scriptable property to correspond to the display style attribute, so you cannot modify this property on the fly (although Navigator does recognize the display attribute when a page loads). But in Internet Explorer 4, you can change the display property on the fly under script control. When you do, the content automatically reflows, closing up any gap left by the "undisplayed" element. This is how some DHTML-driven collapsible menus are created and controlled.

4.2.5. The z-index Attribute

Positioned elements can overlap each other. While overlapping text doesn't usually make for a good page design, overlapping opaque elements, such as images and blocks with backgrounds, can be put to good use, particularly when the elements are under script control. The z-index attribute lets you direct the stacking order (also called the z-order, where Z stands for the third dimension, after X and Y) of elements within a positioning context. The higher the z-index value (values are integers), the closer the element layer is to the user's eye.

Positioned elements—even if their z-index attributes are not specified in their style rules—exist as a group in a plane closer to the user's eye than nonpositioned content. The notable exception to this is Navigator 4's belief that any form element (positioned or otherwise) should exist in a plane in front of positioned elements, regardless of z-index setting. In other words, you cannot obscure a form element behind a positioned element in Navigator 4.

If you do not specify the z-index attribute for any positioned elements in a document, the default stacking order is based on the sequence in which the positioned elements are defined in the HTML source code. Even so, these positioned items are in front of nonpositioned items (except form elements in Navigator 4). Therefore, you need to specify z-index values only when the desired stacking order is other than the natural sequence of elements in the source code.

More commonly, z-index values are adjusted by scripts when a user interacts with maneuverable content (by dragging or resizing), or when a script moves an element as a form of animation. For example, if your page allows dragging of elements (perhaps an image acting as a piece of a jigsaw puzzle), it may be valuable to set the z-index attribute of that element to an arbitrarily high value as the user drags the image. This keeps the image in front of all other positionable puzzle pieces while being dragged (so it doesn't "submarine" and get lost behind other elements). When the user releases the piece, you can reset the z-index attribute to, say, zero to move it back among the pool of other inactive positioned elements.

You cannot interleave elements that belong to different positioning contexts. This is because z-index values are relative only to sibling elements. For example, imagine you have two positioned DIV elements named Div1 and Div2 (see Figure 4.7). Div1 contains two positioned SPAN elements; Div2 contains three positioned SPAN elements. A script can adjust the z-index values of the elements in Div1 all they want, but the two elements are always kept together; similarly the three elements in Div2 are always "contiguous" in their stacking order. If you swap the z-index values of Div1 and Div2, the group of elements contained by each DIV swaps positions as well.

Figure 4.7. Stacking order is relative to the positioning context of the element

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