HTML The Definitive Guide (63 page)

The tag creates a break in the text flow and inserts a blank line before rendering its content into multiple columns. After the tag, another blank line is added and the text flow resumes using the previous layout and formatting.

Netscape automatically balances the columns, making each approximately the same length. Where possible, the browser moves text between columns to accomplish the balancing. In some cases, the columns cannot be perfectly balanced because of embedded images, tables, or other large elements.

You can nest tags, embedding one set of columns within another set of columns.

While infinite nesting is supported, more than two levels of nesting is generally impractical and results in unattractive text flows.

6.2.1.1 The cols attribute

The cols attribute is required by the tag to define the number of columns. If omitted, Netscape creates just one column, as if the tag isn't there at all. You may create any number of columns, but in practice, more than three or four columns make text unreadable on most displays.

The following example creates a three-column layout: Temperature Effects

The effects of cooler weather on the kumquat's ripening process vary based upon the temperature. Temperatures above 28° sweeten the fruit, while four or more hours below 28° will damage the tree. The savvy quat farmer will carefully monitor the temperature, especially in the predawn hours when the mercury dips to its lowest point. Smudge pots and grove heaters may be required to keep the trees warm; many growers will spray the trees with water to create an insulating layer of ice over the fruit and leaves.

If a disastrous frost is predicted, below 20°, the only recourse may be to harvest the fruit early to save it from an assured disaster. Kumquats may subsequently be ripened using any of the popular methane and cyanoacrylate injection systems used for other citrus fruits. Used correctly, these systems will produce fruit whose taste is indistinguishable from tree-ripened kumquats.

The results are shown in
Figure 6.4.

Figure 6.4: A three-column document segment
You can see in Figure 6.4 how Netscape has balanced the columns to approximately equal lengths.

You also can see how several lines within the columns appear shorter, since longer words were wrapped to the next line of text. These overly ragged right margins within the columns are unavoidable and serve to emphasize that you shouldn't create more than four or five columns in a flow. Our example is still barely readable if displayed as five columns; it breaks down completely and even induces rendering errors if cols is set to 7, as shown in
Figure 6.5
.

Figure 6.5: Too many columns create unreadable pages

6.2.1.2 The gutter attribute

The space between columns is known as the
gutter
. By default, Netscape creates a gutter 10 pixels wide between each of your columns. To change this, set the gutter attribute's value to the desired width in pixels. Netscape will reserve this much space between your columns; the remaining space will be used for the columns themselves.

Figure 6.6
shows the effect this can have on your columns. In this figure, we've reformatted our sample text using . Contrast this with
Figure 6.4, which uses

the default 10-pixel gutters.

Figure 6.6: Gutter widths can be changed with the gutter attribute

6.2.1.3 The width attribute

Normally, the tag fills the current width of the current text flow. To have your multiple columns occupy a thinner space, or to extend them beyond the visible window, use the width attribute to specify the overall width of the tag. The columns will be resized so that the
columns plus the gutters fill the width you've specified.[1] The width may be specified as an absolute

number of pixels or as a percentage of the width of the current text flow.

[1] To be exact, each column will be (
w-g
(
n-
1))/
n
pixels wide, where
w
is the width of the tag,
g
is the width of a gutter, and
n
is the number of columns. Thus, using creates columns that are 160 pixels wide.

Figure 6.7
shows the effects of adding width="75%" to our column example, retaining the default gutter width of 10 pixels.

Figure 6.7: Changing the width of the tag

Be careful when you reduce the size of your columns if they include images or other fixed-width elements. Netscape will not wrap text around images that extend beyond the boundaries of a column.

Instead, the image simply covers the adjacent columns, ruining your document. Always make sure that embedded elements in columns are small enough to fit within your columns, even on fairly small browser displays.

6.2.1.4 The style and class attributes
Use the style attribute with the tag to create an inline style for all the content inside the tag. The class attribute lets you label the section with a name that refers to a predefined class of the
tag declared in some document-level or externally defined style sheet. Section

9.1.1, "Inline Styles: The style Attribute" in
Chapter 9
Section 9.2.4, "Style Classes" in
Chapter 9.

6.2.2 Multiple Columns and Other Browsers
As we've noted, the tag is supported only by Netscape. Fortunately, when other browsers encounter the tag, they ignore it and render the enclosed text as part of the normal text flow, usually with little consequent disruption to the document.

The only problem may be that the contents of the tag flow up into the previous flow, without an intervening break. For that reason, you might consider preceding every tag with a

tag. Netscape won't mind, and other browsers will at least perform a paragraph break before rendering your multicolumn text in a single column.

It is possible emulate the tag using tables, but the results are crude and difficult to manage across multiple browsers. To do so, create a single row table with a cell for each column.

Place an appropriate amount of the text flow in each cell to achieve balanced columns. The difficulty, of course, is that the "appropriate amount" varies wildly between browsers, making it almost impossible to create multiple columns that are attractive on several different browsers.