This section contains definitions of terms that appear elsewhere on this website.


An author is a person or program that writes or generates HTML documents.

Client-side Scripting

Client-side scripting generally refers to the category of computer programs on the web that are executed client-side i.e. by the user's web browser. JavaScript is the main client-side scripting language for the Web.

Computed Value

The computed value of a CSS property is computed from the specified values during the cascade. It typically involves the process of converting relative values such as 'em' and 'ex' to pixel or absolute lengths; for example, if an element has specified values font-size:12px and margin:2em, then the computed value of margin is 24px (double the font size).

The computed value exists even when the property does not apply. However, some properties may define the computed value of a property for an element to depend on whether the property applies to that element.

Content Type

A content-type is used to define the nature of a linked resource. This allows the browser to load the linked resource in its appropriate form.

Commonly used content types are "text/css" (indicates a Cascading Style Sheet), "text/javascript" (indicates JavaScript), "text/html", "image/jpeg", "image/gif", "video/mpeg", "audio/basic", etc. Content types are case-insensitive.


A deprecated function, element or attribute is one that has been outdated by newer constructs and may become obsolete in future versions. Therefore it should be avoided.

All deprecated elements and attributes have been removed from the strict versions of HTML i.e. XHTML. However, the browsers may still support the deprecated HTML elements and attributes for the reasons of backward compatibility.

Document Tree

The tree of elements encoded in the source document. Each element in this tree has exactly one parent, with the exception of the root element, which has none.


The 'initial' keyword defined in CSS3 specifications which represents the specified value that is designated as the property's initial value. All properties accept the 'initial' value.


The 'inherit' value is allowed on every CSS property. It causes the element for which it is specified to take the computed value of the property from its parent element.

Animatable as a length, percentage, or calc()

When both values are lengths, they are interpolated as lengths; when both values are percentages, they are interpolated as percentages; otherwise, both values are converted into a calc() function that is the sum of a length and a percentage (each possibly zero), and these calc() functions have each half interpolated as real numbers.

The calc() function allows mathematical expressions with addition ('+'), subtraction ('-'), multiplication ('*'), and division ('/') to be used as component values. The calc() expression represents the result of the mathematical calculation it contains, using standard operator precedence rules. It can be used wherever length, frequency, angle, time, number, or integer values are allowed, for example, div { width: calc(100%/3 - 2*1em - 5*10px); }, div { height: calc(50% + 30px); }, etc.


Nonstandard elements and attributes are not included in any version of the HTML or XHTML Recommendations but are well supported by browsers.

Not in HTML5

Attributes marked Not in HTML5 have been omitted from HTML5, usually because they are presentational or were never supported by popular browsers.


An obsolete element or attribute is one that is no longer supported. Obsolete elements are no longer defined in the specification, but are listed for historical purposes.

Replaced Element

A replaced element is any element whose content is outside the scope of the CSS formatting model, such as images (<img>), plugins (<object>), and form elements (<button>, <textarea>, <input>, and <select>). All other elements types can be referred to as non-replaced elements.

The appearance and dimensions of replaced elements are defined by an external resource; for example, the content of the HTML <img> element is often replaced by the image designated by its "src" attribute.


Attributes marked as Required must be included in the element for the markup to be valid.

The Cascade

The CSS cascade assigns a weight to each style rule. When several rules apply, the one with the greatest weight takes precedence.


A user is a person who interacts with a user agent to view, hear, or otherwise use a rendered HTML document.

User Agent

An HTML user agent is any device that interprets HTML documents.

User agents include visual browsers (text-only and graphical), non-visual browsers (audio, braille), search engines robots or web crawlers, proxies, etc.


A window or other viewing area on the screen through which users see a document.

XHTML only

Attributes marked XHTML only apply only to documents marked up in XHTML 1.0 or 1.1. Other minor differences between HTML and XHTML are noted similarly.

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