Object Element

An HTML element is a type of HTML (HyperText Markup Language) document component, one of several types of HTML nodes (there are also text nodes, comment nodes and others). The first used version of HTML was written by Tim Berners-Lee in 1993 and there have since been many versions of HTML. The most commonly used version is HTML 4.01, which became official standard in December 1999. An HTML document is composed of a tree of simple HTML nodes, such as text nodes, and HTML elements, which add semantics and formatting to parts of document (e.g., make text bold, organize it into paragraphs, lists and tables, or embed hyperlinks and images). Each element can have HTML attributes specified. Elements can also have content, including other elements and text.

Concepts

Elements vs. tags

As is generally understood, the position of an element is indicated as spanning from a start tag and is terminated by an end tag. This is the case for many, but not all, elements within an HTML document. The distinction is explicitly emphasised in ''HTML 4.01 Specification'':

Similarly the W3C Recommendation ''HTML 5.1 2nd Edition'' explicitly says:

and:

As HTML (before HTML5) is based on SGML, its parsing also depends on the Document Type Definition (DTD), specifically an HTML DTD (e.g. HTML 4.01). The DTD specifies which element types are possible (i.e. it defines the set of element types) and also the valid combinations in which they may appear in a document. It is part of general SGML behavior that, where only one valid structure is (per the DTD), its explicit statement in any given document is not generally required. As a simple example, the tag indicating the start of a paragraph element should be complemented by a tag indicating its end. But since the DTD states that paragraph elements cannot be nested, an HTML document fragment is thus inferred to be equivalent to . (If one paragraph element cannot contain another, any currently open paragraph must be closed before starting another.) Because this implication is based on the combination of the DTD and the individual document, it is not usually possible to infer elements from document tags but only by using an SGML—or HTML—aware parser with knowledge of the DTD. HTML5 creates a similar result by defining what tags can be omitted.

SGML vs. XML

SGML is complex, which has limited its widespread understanding and adoption. XML was developed as a simpler alternative. Although both can use the DTD to specify the supported elements and their permitted combinations as document structure, XML parsing is simpler. The relation from tags to elements is always that of parsing the actual tags included in the document, without the implied closures that are part of SGML.

HTML as used on the current web is likely to be either treated as XML, by being XHTML, or as HTML5; in either case the parsing of document tags into Document Object Model (DOM) elements is simplified compared to legacy HTML systems. Once the DOM of elements is obtained, behavior at higher levels of interface (example: screen rendering) is identical or nearly so.

%block; vs. box

Part of this CSS presentation behavior is the notion of the " box model". This is applied to those elements that CSS considers to be "block" elements, set through the CSS declaration.

HTML also has a similar concept, although different, and the two are very frequently confused. %block; and %inline; are groups within the HTML DTD that group elements as being either "block-level" or "inline". This is used to define their nesting behavior: block-level elements cannot be placed into an inline context. This behavior cannot be changed; it is fixed in the DTD. Block and inline elements have the appropriate and different CSS behaviors attached to them by default, including the relevance of the box model for particular element types.

Note though that this CSS behavior can, and frequently is, changed from the default. Lists with are %block; elements and are presented as block elements by default. However, it is quite common to set these with CSS to display as an inline list.

Overview

Syntax

In the HTML syntax, most elements are written with a start tag and an end tag, with the content in between. An HTML tag is composed of the name of the element, surrounded by angle brackets. An end tag also has a slash after the opening angle bracket, to distinguish it from the start tag. For example, a paragraph, which is represented by the element, would be written as:

In the HTML syntax, most elements are written ...

However, not all of these elements the end tag, or even the start tag, to be present. Some elements, the so-called ''void elements'', do not have an end tag. A typical example is the (hard line-break) element. A void element's behavior is predefined, and it cannot contain any content or other elements. For example, an address would be written as:

P. Sherman
42 Wallaby Way
Sydney

When using XHTML, it is required to open and close all elements, including void elements. This can be done by placing an end tag immediately after the start tag, but this is not legal in HTML 5 and will lead to two elements being created. An alternative way to specify that it is a void element, which is compatible with both XHTML and HTML 5, is to put a / at the of the tag (not to be confused with the / at the of a closing tag).

P. Sherman
42 Wallaby Way
Sydney

HTML attributes are specified inside the start tag. For example, the element, which represents an abbreviation, expects a title attribute within its opening tag. This would be written as:

abbr.

Informally, HTML elements are sometimes referred to as "tags" (an example of synecdoche), though many prefer the term ''tag'' strictly in reference to the markup delimiting the start and end of an element.

Element (and attribute) names may be written in any combination of upper or lower case in HTML, but must be in lower case in XHTML.XHTML 1.0 §4.2 The canonical form was upper-case until HTML 4, and was used in HTML specifications, but in recent years, lower-case has become more common.

Types of element

There are three kinds of HTML elements: normal elements, raw text elements, and void elements.

usually have both a start tag and an end tag, although for some elements the end tag, or both tags, can be omitted. It is constructed in a similar way:
* a ''start tag'' () marking the beginning of an element, which may incorporate any number of HTML attributes;
* some amount of ''content'', including text and other elements;
* an ''end tag'', in which the element name is prefixed with a slash: .

(also known as text or text-only elements) are constructed with:
* a ''start tag'' (in the form ) marking the beginning of an element, which may incorporate any number of HTML attributes;
* some amount of text ''content'', but no elements (all tags, apart from the applicable end tag, will be interpreted as content);
* an ''end tag'', in which the element name is prefixed with a slash: . In some versions of HTML, the end tag is optional for some elements. The end tag is required in XHTML.
An example is the element, which must not contain other elements (including markup of text), only text.

(also sometimes called empty elements, single elements or stand-alone elements) only have a start tag (in the form ), which contains any HTML attributes. They may not contain any children, such as text or other elements. For compatibility with XHTML, the HTML specification allows an optional space and slash ( is permissible). The slash is required in XHTML