Section 12.4. JavaScript Style Sheets (Antiquated)

Much of a browser's work is manipulating the display, and much of its display code already has been exposed for JavaScripting. So it seemed only natural, perhaps even relatively easy, for the developers at Netscape to implement JavaScript Style Sheets ( JSS). Based on the W3C-recommended Cascading Style Sheet (CSS) model, outlined in Chapter 8, this alternative document style technology lets you prescribe display properties for all the various HTML elements, either inline as tag attributes, at the document level, or for an entire document collection.

JSS is a Netscape invention. In fact, for a short time, Netscape appeared ready to eschew the CSS methodology, which Internet Explorer already had implemented, and use JSS exclusively for HTML document designers with its then-current browser, Navigator 4. In the end, Netscape supported both JSS and CSS technologies. Today, Netscape 6 eschews support for JSS entirely in favor of the standard CSS2. At this point, CSS should be seen as Netscape's long-term direction.

We are strong proponents of reasonable standards, and now that the CSS2 model is fully supported in HTML 4 and XHTML, we can't recommend that you use anything but CSS-standard style sheets. Evidently, Netscape now agrees with us on this point.

We thoroughly discuss the concepts and ideas behind style sheets ? specifically, Cascading Style Sheets ? in Chapter 8, so we won't repeat ourselves here. Rather, we address only how to create and manipulate styles with JavaScript (purely for historical reasons, since no current browser supports them). Before forging ahead in this section, we recommend that you first absorb the information in Chapter 8.

12.4.1 JavaScript Style Sheet Syntax

Netscape Versions 4 and earlier implement JSS by extending several existing HTML tags and defining a few objects that store your document's styles. Netscape 6 no longer supports JSS.

12.4.1.1 External, document-level, and inline JSS

As with CSS, you can reference and load external JSS files with the <link> tag. For example:

<link href="styles.js" rel=stylesheet type=text/JavaScript>

The only real difference between this tag and the one for a CSS external style sheet is that the type attribute of the <link> tag is set to text/JavaScript instead of text/CSS. The referenced file, styles.js, contains JavaScript statements that define styles and classes that Netscape then uses to control display of the current document.

Document-level JSS is defined within a <style> tag in the <head> of the document, just like with CSS. Again, there is only one real difference: the type attribute of the <style> tag is set to text/JavaScript instead of text/CSS.

The contents of the <style> tag for JSS are quite different from those for CSS, however. For example:

<style type=text/JavaScript>

<!--

    tags.BODY.marginLeft = "20px";

    tags.P.fontWeight = "bold";

  // -->

</style>

First, notice that we use the standard JavaScript and HTML comments to surround our JSS definitions, preventing noncompliant browsers from processing them as HTML content. Also notice that the syntax of the style definition is that of JavaScript, where letter case, among other things, does make a difference.

You associate inline JavaScript-based style rules with a specific tag using the style attribute, just like with CSS inline styles. The value of the attribute is a list of JSS assignments, separated by semicolons. For example:

<p style="color = 'green'; fontWeight = 'bold'">

creates a green, bold-faced text paragraph. Notice first that you need to enclose inline style values within single quotation marks, not double quotation marks, as you might use for document-level and external JSS styles. This is reasonable, since the style attribute value itself must be enclosed in double quotation marks.

Also note that inline JSS definitions use only the property name, not the containing tag object that owns the property. This makes sense, since inline JSS styles affect only the current tag, not all instances of the tag.

12.4.1.2 JSS values

In general, all of the values you may use for CSS may also be used in JSS definitions. For keyword, length, and percentage values, simply enclose the value in quotes and use it as you would any string value in JavaScript. Thus, the CSS value bold becomes "bold" or 'bold' for JSS document-level or inline styles, respectively; 12pt in CSS becomes '12pt' or "12pt" in JSS.

Specify color values as the color name or a hexadecimal color value, enclosed in single or double quotes. The CSS decimal RGB notation is not supported in JSS.

JSS URL values are strings containing the desired URL. Thus, the CSS URL value url(http://www.kumquat.com) becomes 'http://www.kumquat.com' for a JSS inline style, or "http://www.kumquat.com" at the document level.

One unique power of JSS is that any value can be computed dynamically when the document is processed by the browser. Instead of statically specifying the font size, for example, you can compute it on the fly:

tags.P.fontSize = favorite_font_size(  );

We assume that the JavaScript function favorite_font_size( ) somehow determines the desired font size and returns a string value containing that size. This, in turn, is assigned to the fontSize property for the <p> tag, defining the font size for all paragraphs in the document.

12.4.1.3 Defining styles for tags

JavaScript defines a document property called tags that contains the style properties for all HTML tags. To define a style for a tag, simply set the appropriate property of the desired style property within the tag property of the document object. For example:

document.tags.P.fontSize = '12pt';

document.tags.H2.color = 'blue';

These two JSS definitions set the font size for the <p> tag to 12 points and render all <h2> tags in blue. The equivalent CSS definitions are:

p {font-size : 12pt}

h2 {color : blue}

Since the tags property always refers to the current document, you may omit document from any JSS tag style definition. We could have written the previous two styles as:

tags.P.fontSize = '12pt';

tags.H2.color = 'blue';

Moreover, as we mentioned previously, you may omit the tag name, as well as the document and tags properties for inline JSS using the style attribute.

Capitalization and case are significant in JSS. The tag names within the tags property must always be fully capitalized. The embedded capital letters within the tag properties are significant: any deviation from the exact lettering produces an error, and Netscape won't honor your JSS declaration. All of the following JSS definitions are invalid, though the reasons are not overly apparent:

tags.p.fontsize = '12pt';

tags.Body.Color = 'blue';

tags.P.COLOR = 'red';

The correct versions are:

tags.P.fontSize = '12pt';

tags.BODY.color = 'blue';

tags.P.color = 'red';

It can be very tedious to specify a number of properties for a single tag, so you can take advantage of the JavaScript with statement to reduce your typing burden. These styles:

tags.P.fontSize = '14pt';

tags.P.color = 'blue';

tags.P.fontWeight = 'bold';

tags.P.leftMargin = '20%';

can more easily be written as:

with (tags.P) {

  fontSize = '14pt';

  color = 'blue';

  fontWeight = 'bold';

  leftMargin = '20%';

  }

You can apply similar styles to diverse tags just as easily:

with (tags.P, tags.LI, tags.H1) {

  fontSize = '14pt';

  color = 'blue';

  fontWeight = 'bold';

  leftMargin = '20%';

  }

12.4.1.4 Defining style classes

Like CSS, JSS lets you target styles for specific ways that a tag can be used in your document. JSS uses the classesproperty to define separate styles for the same tag. There are no predefined properties within the classes property; instead, any property you reference is defined as a class to be used by the current document. For example:

classes.bold.P.fontWeight = 'bold';

with (classes.abstract.P) {

  leftMargin = '20pt';

  rightMargin = '20pt';

  fontStyle = 'italic';

  textAlign = 'justify';

  }

The first style defines a class of the <p> tag named bold whose font weight is set to bold. The next style uses the with statement to create a class of the <p> tag named abstract with the specified properties. The equivalent CSS rules would be:

P.bold {font-weight : bold}

P.abstract {left-margin : 20pt;

  right-margin : 20pt;

  font-style : italic;

  text-align : justify

  }

Once defined, use a JSS class just like any CSS class: with the class attribute and the class name.

Like CSS, JSS also lets you define a class without defining the tag that uses the class. This lets you define generic classes that you can later apply to any tag. To create a generic style class in JSS, use the special tag property all:

classes.green.all.color = "green";

You can then add class="green" to any tag to have Netscape render its contents in green. The equivalent CSS is:

.green {color : green}

12.4.1.5 Using contextual styles

One of the most powerful aspects of CSS is its contextual style capability, wherein the browser applies a style to tags only if they appear in the document in a certain nesting. JSS supports contextual styles as well, through the special contextual( ) method within the tags property. The parameters to this method are the tags and classes that define the context in which Netscape applies the style. For example:

tags.contextual(tags.UL, tags.UL, tags.LI).listStyleType = 'disc';

defines a context wherein the elements (tags.LI) of an unordered list nested within another unordered list (tags.UL, tags.UL) use the disc as their bullet symbol. The CSS equivalent is:

ul ul li {list-style-type : disc}

You can mix tags and classes in the contextual( ) method. For instance:

tags.contextual(classes.abstract.P, tags.EM).color = 'red';

tells the browser to display in red <em> tags that appear within paragraphs that are of the abstract class. The CSS equivalent is:

p.abstract em {color : red}

Since the tags object is unambiguously included within the contextual( ) method, you may omit it from the definition. Hence, our nested list example may be rewritten as:

tags.contextual(UL, UL, LI).listStyleType = 'disc';

12.4.2 JavaScript Style Sheet Properties

A subset of the CSS style properties are supported in JSS. The JSS style properties, their CSS equivalents, and the sections in which those properties are fully documented are shown in Table 12-2.

Table 12-2. JSS properties and CSS equivalents
JSS property	CSS property	See section
`align`	`float`	Section 8.4.7.9
`backgroundImage`	`background-image`	Section 8.4.5.3
`backgroundColor`	`background-color`	Section 8.4.5.2
`borderBottomWidth`	`border-bottom-width`	Section 8.4.7.4
`borderLeftWidth`	`border-left-width`	Section 8.4.7.4
`borderRightWidth`	`border-right-width`	Section 8.4.7.4
`borderStyle`	`border-style`	Section 8.4.7.5
`borderTopWidth`	`border-top-width`	Section 8.4.7.4
`clear`	`clear`	Section 8.4.7.7
`display`	`display`	Section 8.4.10.1
`fontSize`	`font-size`	Section 8.4.3.2.
`fontStyle`	`font-style`	Section 8.4.3.5
`height`	`height`	Section 8.4.7.10
`lineHeight`	`line-height`	Section 8.4.6.2
`listStyleType`	`list-style-type`	Section 8.4.8.3
`marginBottom`	`margin-bottom`	Section 8.4.7.11
`marginLeft`	`margin-left`	Section 8.4.7.11
`marginRight`	`margin-right`	Section 8.4.7.11
`marginTop`	`margin-top`	Section 8.4.7.11
`paddingBottom`	`padding-bottom`	Section 8.4.7.12
`paddingLeft`	`padding-left`	Section 8.4.7.12
`paddingRight`	`padding-right`	Section 8.4.7.12
`paddingTop`	`padding-top`	Section 8.4.7.12
`textDecoration`	`text-decoration`	Section 8.4.6.4
`textTransform`	`text-transform`	Section 8.4.6.7
`textAlign`	`text-align`	Section 8.4.6.3
`textIndent`	`text-indent`	Section 8.4.6.5
`verticalAlign`	`vertical-align`	Section 8.4.6.7
`whiteSpace`	`white-space`	Section 8.4.10.2
`width`	`width`	Section 8.4.7.16

JSS also defines three methods that allow you to define margins, padding, and border widths within a single style property. The three methods, margins( ), paddings( ), and borderWidths( ), accept four parameters, corresponding to the top, right, bottom, and left margin, padding, or border width, respectively. Unlike their CSS counterparts (margin, discussed in Section 8.4.7.11; padding, discussed in Section 8.4.7.12; and border-width, discussed in Section 8.4.7.4), these JSS methods require that you always specify all four parameters. There is no shorthand way in JSS to set multiple margins, paddings, or border widths with a single value.