Applying style to pages Contents

Applying style to pages Contents

Declarations

Declarations are used to apply style to elements, for example, to set an element to have a border.

The basic syntax of a declaration is a property (such as color) followed by a colon (:) and a value. Thus color: red is a declaration. Declarations are directly used only in inline style.

Declaration grouping

In order to specify several styles for an element, declarations can be separated by semi-colons. For example, <P style="color: red; font-size: 16px;">. It is optional to have a semi-colon after the final declaration - <P style="color: red; font-size: 16px"> is also valid.

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Selectors

Selectors are used to associate style declarations with an element or elements.

This is done by placing the declarations within a block (enclosed in {}) and preceding it with a selector. For example, to associate the color: red declaration with all P elements, you would place that declaration in a block and add a selector that matches P:

P {color: red}

The example below would make all 'DIV' elements red and 16 pixels high:

DIV {color: red;
font-size: 16px}

At its most basic, a selector consists of an element name (for example, <P>) (without the < and >). Selectors such as P or A:link are called simple.

Selectors

The type selector

BODY

The CSS selector involves taking the HTML element, removing the < and >. It is used, as previously explained, in an example such as BODY {color: red}.

Class selectors

These allow you to group elements in a class. For example, (if you've got CSS in your browser) you will see all the code exerpts have a gray background with black border. This is due to a class.

The HTML code for a class looks like this <TAG class="classname">. For example:

<P class="introductoryparagraph"> .... </P>

These are distinguished in the style sheet by the use of a period followed by the class name:

P.introductoryparagraph {color: blue}

This applies to P elements with a class of introductorypargraph.

.introductoryparagraph {color: blue}

This would apply to any element with a class of introductoryparagraph (the other one would only apply to P elements).

Class names should describe the content of the element - instead of class="italic" you should say why you want it to be italic, e.g. class="copyrightnotice". That way your style sheet will be self-documenting, and you can easily change appearance according to element content.

Classes may not start with a number or hyphen, and should be in lowercase (because although all browsers should distinguish between lower and upper case, most do not). They may not contain spaces in their names. They may contain any letter of the alphabet as well as numbers and hyphens (but not at the start of the class).

An element may have more than one class, e.g., <P class="green quote new"> matches, e.g., P.quote.green, P.new or P.quote.new.green, but not P.new.old. However, few browsers support this use.

Pseudo-class selectors

These are so called because the behavior is as if the element was specially marked as that class.

Note that pseudo-class selectors that cause the document to reflow may be ignored by web browsers. An example of this would be a different font size on a visited link to an unvisited one.

:link, :visited

These apply to unvisited, and visited links respectively. A link cannot be both visited and unvisited. E.g.:

A:link {color: red} /* :link would be the same as A:link, because there aren't any other elements to which :link applies */
A:visited {color: purple}

:hover and :focus selectors

Hover applies when a mouse is over an element, and focus when the element has the focus; e.g., a value being typed in a form field. For example, P:hover {text-decoration: underline}, or A:focus. Hover and focus are not mutually exclusive with each other or with :link and :visited.

:active selectors

This applies to an element being activated. For example, INPUT:active {color: red} would make INPUT elements in the process of being activated red.

Links can be active as well as linked or visited. In addition, :active applies to all elements - not just links.

Pseudo-classes on links

Consider:

A:active {background-color: red}
A:hover {background-color: yellow}
A:link {background-color: green}

Presumably the intention would be to have unvisited links green, except when a mouse is hovering over it, when the desired background color is yellow, and when it is being activated, when the background color should be red.

However, this would not happen, because the states are not mutually exclusive, and since A:link comes last, it overrides the previous ones.

In order to specify more about an element, you can use multiple pseudo-classes; for example, A:link:hover. Thus to specify that you want hovered over unvisited links with the focus to be red, you would say A:link:hover:focus {color: red}. Note that only Netscape 6, and not Windows Internet Explorer, supports this usage.

Contextual selectors

These provide that if ELEMENT2 is a descendant of ELEMENT1, then the given properties apply to those ELEMENT2s. E.g.:

STRONG EM {text-transform: uppercase} /* EM inside STRONG will be uppercase */

This is a very important concept indeed. To take an example, the links in the contents tables at the top of this page are enclosed in <DIV class="contents"></DIV>. That means that to make the links a different color you can just type DIV.contents A {color: blue}, which is equivalent to specifying that you want all links (all A elements) inside the DIV to be blue.

Selector grouping

In order to associate a block with several different selectors, and therefore save typing and download time, one can separate the selectors by commas.

BODY P, H1 {color: red}

is the same as:

BODY P {color: red}
H1 {color: red}