Difference between a pseudo-class and a pseudo-element

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Pseudo-Class

A pseudo-class is way of selecting certain parts of a HTML document, based in principle not on the HTML document tree itself and its elements or on characteristics like name, attributes or contents, but on other phantom conditions like language encoding or the dynamic state of an element.

The original pseudo-class defined dynamic states of an element that are entered and exited over time, or through user intervention. CSS2 expanded on this concept to include virtual conceptual document components or inferred portions of the document tree e.g. first-child. Pseudo-classes operate as if phantom classes were added to various elements.

RESTRICTIONS: Unlike pseudo-elements, pseudo-classes can appear anywhere in selector chain.

a:link /* This selects any "a" element whose target has not been visited.*/
{
padding : 4px;
text-decoration : none;
width : 10%;
color : #000000; /* black text color */
background-color : #99FF99; /* set to a pastel green */
border-top : 2px solid #ccffcc; /* highlight color */
border-left : 2px solid #ccffcc; /* highlight color */
border-bottom : 2px solid #003300; /* shadow color */
border-right : 2px solid #003300; /* shadow color */
}

a:visited /* This selects any "a" element whose target has been visited.*/
{ padding : 4px;
text-decoration : none;
color : #000000; /* black text color */
background-color : #ccccff; /* set to a lavender */
border-top : 2px solid #ffffff; /* highlight color */
border-left : 2px solid #ffffff; /* highlight color */
border-bottom : 2px solid #333366; /* shadow color *
border-right : 2px solid #333366; /* shadow color */
}

a:hover /* This selects any "a" element which is in a hover state. This is a state during pointer movement within the rendering region of an element. The user designates an element but does not activate it. */
{
color : #000000; /* black text color */
background-color : #99cc99; /* desaturated color */
border-top : 2px solid #003300; /* shadow color */
border-left : 2px solid #003300; /* shadow color */
border-bottom : 2px solid #ccffcc; /* highlight color */
border-right : 2px solid #ccffcc; /* highlight color */
}

a:focus /* This selects any "a" element which currently has focus. Focus is a state during which an element accepts keyboard input or other forms of text input. */
{
padding : 4px;
text-decoration : none;
width : 10%;
color : #000000; /* black text color */
background-color : #ffff99; /* set to a pastel yellow */
border-top : 2px solid #ffffcc; /* highlight color */
border-left : 2px solid #ffffcc; /* highlight color */
border-bottom : 2px solid #666633; /* shadow color */
border-right : 2px solid #666633; /* shadow color */
}

a:active /* This selects any "a" element which is in a state of activation. Active is a state during pointer activation (eg: press and release of a mouse) within the rendering region of an element.*/
{
padding : 4px;
text-decoration : none;
width : 10%;
color : #000000; /* black text color */
background-color : #ff99ff; /* set to a pink */
border-top : 2px solid #ffccff; /* highlight color */
border-left : 2px solid #ffccff; /* highlight color */
border-bottom : 2px solid #663366; /* shadow color */
border-right : 2px solid #663366; /* shadow color */
}

Pseudo-elements

PSEUDO-ELEMENTS are used to address sub-parts of elements. They allow you to set style on a part of an element's content beyond what is specified in the documents. In other words they allow logical elements to be defined which are not actually in the document element tree. Logical elements allow implied semantic structure to be addressed in CSS selectors.

RESTRICTIONS: Pseudo-elements may only be applied to external and document-level contexts - not to in-line styles. Pseudo-elements are restricted in where they can appear in a rule. They may only appear at the end of a selector chain (after the subject of the selector). They should come after any class or ID names found in the selector. Only one pseudo-element can be specified per selector. To address multiple pseudo-elements on a single element structure, multiple style selector/declaration statements must be made.

Pseudo-elements can be used for common typographic effects such as initial caps and drop caps. They can also address generated content that is not in the source document (with the "before" and "after") An example style sheet of some pseudo-elements with properties and values added follows.

/* The following rule selects the first letter of a heading 1 and sets the font to 2em, cursive, with a green background. First-letter selects the first rendered letter/character for a block-level element. */

h1:first-letter {
font-size : 2em;
font-family : "Lucida Handwriting", "Lucida Sans", "Lucida Console", cursive;
background-color : #ccffcc;
}

/* The following rule selects the first displayed line in a paragraph and makes it bold. First-line selects the first rendered line on the output device of a block-level element. */

p:first-line {
font-weight : bold;
}

/* The following rule selects any content placed before a blockquote and inserts the phrase "Quote of the day:" in bold small caps with a green background. */

blockquote:before {
content : "Quote of the day:";
background-color : #ccffcc;
font-weight : bold;
font-variant : small-caps;
}

/* The following rule selects any content placed before a "q" element and inserts the smart open quote. */

q:before {
content : open-quote;
}

/* The following rule selects any content placed after a "q" element and inserts the smart close quote. */

q:after{
content : close-quote;
}