© 2004, robert k. moniot chapter 6 css : cascading style sheets

© 2004, Robert K. Moniot

Chapter 6

CSS : Cascading Style Sheets


Style Sheets

• Each element on a page has a style defined for it.• The style is defined by a set of attribute : value pairs.• Style attributes can control:

– Typeface and font properties– Background properties– Box-related properties– List properties


Ways to define styles

• Default style: provides values for all element properties, unless you change it. (Note: user can customize browser to change defaults!)

• Inline style: style is defined as an attribute of the element in-place. Use this for “one-off” or special styles.

• Embedded style sheet: styles defined in the head portion of web page. Use this if you don’t have very many web pages, or for styles that are useful only for the given page.

• External style sheet: styles defined in a separate file. Use this to centralize style definitions and provide uniformity across all pages of a web site.


Embedded Style Sheet<html><head><title>Page with embedded style</title><style type="text/css"> selector { attribute : value ; attribute : value ... } selector { attribute : value ; attribute : value ... } ...</style></head> ...</html>

•Style definitions go into a <style> element in document head.•Selector determines what elements the style rule applies to.•Style definitions separated by ; are enclosed in { }


Embedded Style Sheet (cont’d)

<html><head><title>Page with embedded style</title><style type="text/css"> selector { attribute : value ; attribute : value ... } selector { attribute : value ; attribute : value ... } ...</style></head> ...</html>

•The type attribute can only be "text/css". (It is leaving room for future alternative style languages.)•Note: CSS is not HTML!


Example<html><head><title>Example page with embedded style</title><style type="text/css"> body { font-family : sans-serif; color : blue; background-color : yellow } h1 { font-style : italic } p { font-size : 14pt } ol { font-size : 12pt; color : red; font-family : serif }</style></head> ...</html>

Here the selectors are simply tag names. The style rules will apply to elements defined by those tags. Result (Example 1)


Inheritance

• A descendant is an element that is enclosed (nested) in another, its ancestor. (If it is an immediate descendant, it is a child of the enclosing element, its parent. Elements having the same parent are siblings.)

• All descendants of an element inherit its style properties, unless these are overridden by their own style rules.

• If two styles could apply to the same element, the one defined by the more specific rule will be used. For instance, an explicit rule is always more specific than an inherited rule.


Compound Selectors

• Selectors can be defined so that a style rule applies to an element only when it is a descendant of a certain other type of element. Examples:

ul ul { list-style-type : square }This specifies that an unordered list inside another unordered list will be bulleted by squares.

h1 em em { color : red }This specifies that emphasized text inside emphasized text in an <h1> header will appear in red.


Compound selectors are more specific than simple selectors. For instance, if a style sheet defines both

p { color : red } div p { color : blue }

then for a <p> tag that is inside a <div> element, the second rule would apply.

Compound Selectors


Style Classes• These allow you to control which elements of a given

type should use a style rule. This method has two parts:– In the style sheet, the selector defines the class name, which is

preceded by a period.– In the HTML, the tag includes the class attribute and specifies

the value of the class name

Example:

Define the nodec class for anchor tags:

a.nodec { text-decoration : none }

This suppresses the usual underlining. Use it in HTML like so:

<a class="nodec" href="somepage.html">Link text</a>


Style Classes

• Style classes can also be “generic,” i.e. not tied to a specific element type. Example:

Define the zowie class:

.zowie { text-decoration : blink }

Use it on an emphasized element:

<em class="zowie">Important!</em>

Use it with no other style attributes:

<span class="zowie">Buy Now!</span>

By the way: promise me you won’t ever use blink!

Example 2


The <span> and <div> Tags

• These tags are provided to allow arbitrary chunks of HTML to be treated as elements. This is usually done in order to apply style attributes to them, as in the preceding example.

• A <span> ... </span> element defines an “inline” structure, i.e. it simply defines a stretch of text. Thus it can be used within a paragraph or table element without affecting the flow of the text.

• A <div> ... </div> element defines a “block” structure. Usually the browser will place line breaks before and after this element, but otherwise it has no effect itself.


Pseudo-classes

• These are like style classes, but an element acquires a pseudo-class by user action or by a relationship other than descendancy.

• In the style sheet, a pseudo-class name is preceded by a colon.

• In the HTML, the pseudo-class name is NOT used with the affected tag, because it is implied.


Pseudo-classes

• Link-related pseudo-classes

a:link { color : red }Applies when the link has not yet been visited.

a:visited { color : green }Applies when the link has been visited.

a:hover { color: yellow }Applies when the mouse is over the link.


Cascading Rule

• If two equally specific rules can apply to the same element, the one that comes last in the style sheet is used. Thus, in the example below, a:hover must follow a:link and a:visited in order to have effect, since a link can be both visited (or not) and hovering. The order of the first two doesn’t matter since they are mutually exclusive.

a:link { color : red }a:visited { color : green }a:hover { color : yellow }

Result (Example 3)


Pseudo-elements

• :first-line is a pseudo-element that consists of the first line of text in a block-level element.

• :first-letter is a pseudo-element that consists of the first letter of the text in an element.

Closely related to pseudo-classes, in that they are defined by relationships, not by explicit declarations. A pseudo-element refers to a virtual element that is part of an actual element, rather than a special case of a whole element.


Pseudo-elements

p { text-indent: 1em }p.initial { text-indent: 0 } p.initial:first-line { text-transform: uppercase }p.initial:first-letter { font-size: 24pt }

This indents all normal paragraphs. A paragraph that is declared with class="initial" is not indented, and its first line appears in all capital letters, with an extra-large first letter.

Result (Example 4)


Inline Styles

• Defined for individual elements, at the point of use (in the HTML).

• Useful for “one-off” styles that are not likely to be used elsewhere.

• Method:

<tag style="attribute:value; attribute:value ...">HTML text</tag>

The attribute:value pairs are what would go between { } if this were a style-sheet rule. There is no selector since the style applies to this element only.

Example 4


External Style Sheets

• A style sheet can be placed in a separate file (usually named with suffix .css) and referenced by HTML files that need it.

• Useful for centralizing control of style and ensuring uniform appearance across many pages of a web site.

• The contents of the file are what would go between <style> ... </style> of an embedded style sheet. Note: this file is not HTML!

• The file is referenced using a <link> tag in the HTML document's head portion.


Example of style sheet

/* This style sheet defines an "excerpt" class for paragraphs that is much like blockquote. */

p.excerpt { font-style : italic; margin-right : 2em; margin-left : 2em; }

Here is what an external style sheet (named style.css) could contain:

Note that there is no HTML in this file! This example also illustrates a CSS comment between /* and */ . Such comments can be placed in external or embedded style sheets.


Example of using style sheet

• The rel attribute specifies the relationship of the referenced file to this page.

• The type attribute must be "text/css".• The href attribute is a URL pointing to the external style sheet.

<html><head><title>Style Example</title><link rel="stylesheet" type="text/css" href="style.css"></head> ... <p class="excerpt">affected text</p> ...</html>

Result (Example 5)

© 2004, robert k. moniot chapter 6 css : cascading style sheets

Documents

style properties

style definitions

style rules

style attributes

inline style

embedded style selector

moniot style sheets

embedded style body