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    * HTML element *

    عنصر اچ‌تی‌ام‌ال


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    (Wikipedia) - HTML element This article is about the HTML elements in general. For information on how to format Wikipedia entries, see Help:Wiki markup and Help:HTML in wikitext. "nobr" redirects here. For the chemical compound NOBr, see Nitrosyl bromide. HTML Comparisons
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    An HTML element is an individual component of an HTML document or web page, once this has been parsed into the Document Object Model. HTML is composed of a tree of HTML elements and other nodes, such as text nodes. Each element can have HTML attributes specified. Elements can also have content, including other elements and text. HTML elements represent semantics, or meaning. For example, the title element represents the title of the document.

    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 p element, would be written as

    <p>In the HTML syntax, most elements are written ...</p>

    However, not all of these elements require 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 br element, which represents a significant line break, such as in a poem or an address. A void element''s behaviour is predefined, and it can not contain any content or other elements. For example, the address of the dentist in the movie Finding Nemo would be written as

    <p>P. Sherman<br>42 Wallaby Way<br>Sydney</p>

    When using an XHTML DTD, it is required to open and close the element with a single tag. To specify that it is a void element, a "/" is included at the end of the tag (not to be confused with the "/" at the beginning of a closing tag).

    <p>P. Sherman<br/>42 Wallaby Way<br/>Sydney</p>

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

    <abbr title="abbreviation">abbr.</abbr>

    Contents

    Concepts Document vs. DOM

    HTML documents are delivered as "documents". These are then parsed, which turns them into the Document Object Model (DOM) internal representation, within the web browser.

    Presentation by the web browser, such as screen rendering or access by JavaScript, is then performed on this internal model, not the original document.

    Early HTML documents, and to a lesser extent today, were largely invalid HTML and riddled with syntax errors. The parsing process was also required to "fix-up" these errors, as best it could. The resultant model was often not correct (i.e. it did not represent what a careless coder had originally intended), but it would at least be valid, according to the HTML standard. A valid model was produced, no matter how bad the "tag soup" supplied had been. Only in the rarest cases would the parser abandon parsing altogether.

    Elements vs. tags

    "Elements" and "tags" are terms that are widely confused. HTML documents contain tags, but do not contain the elements. The elements are only generated after the parsing step, from these tags.

    As is generally understood, the position of an element is indicated as spanning from a start tag, possibly including some child content, and is terminated by an end tag. This is the case for many, but not all, elements within an HTML document.

    As HTML is based on SGML, its parsing also depends on the use of a DTD, specifically an HTML DTD such as that for HTML 4.01. The DTD specifies which element types are possible (i.e. it defines the set of element types that go to make up HTML) and it also specifies the valid combinations in which they may appear in a document. It is part of general SGML behaviour that where only one valid structure is possible (per the DTD), it is not generally a requirement that the document explicitly states that structure. As a simple example, the <p> start tag indicating the start of a paragraph element should be closed by a </p> end tag, indicating the end of the element. Also the DTD states that paragraph elements cannot be nested. The HTML document fragment:

    <p>Para 1 <p>Para 2 <p>Para 3

    can thus be inferred to be equivalent to:

    <p>Para 1 </p><p>Para 2 </p><p>Para 3

    (If one paragraph element cannot contain another, any currently open paragraph must be closed before starting another.)

    Because of this implied behaviour, based on the combination of the DTD and the individual document, it is not possible to infer elements from the document tags alone, but only by also using an SGML or HTML aware parser, with knowledge of the DTD.

    SGML vs. XML

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

    Where HTML can be formed as XML, either through XHTML or through HTML5 as XML, the parsing from document tags to DOM elements is simplified, but still follows the same basic process. Once the DOM of elements is obtained, behaviour beyond that point (i.e. screen rendering) is identical.

    %block; vs. box

    Part of this CSS presentation behaviour is the notion of the "box model". This is applied to those elements that CSS considers to be "block" elements, set through the CSS display: block; statement.

    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 behaviour: block-level elements cannot be placed into an inline context. This behaviour cannot be changed, it is fixed in the DTD. Block and inline elements have the appropriate and different CSS behaviours attached to them by default, including the relevance of the box model for particular element types.

    Note though that this CSS behaviour can, and frequently is, changed from the default. Lists with <ul><li> ... 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 SyntaxParts of an HTML container element:

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

    Void elements only have a start tag, which contains any HTML attributes. They may not contain any children, such as text or other elements. Often they are place holders for elements which reference external files, such as the image (<img/>) element. The attributes included in the element will then point to the external file in question. Another example of a void element is the link element, for which the syntax is

    <link rel="stylesheet" href="fancy.css" type="text/css">

    This link element points the browser at a style sheet to use when presenting the HTML document to the user. Note that in the HTML syntax, attributes don''t have to be quoted. When using the XML syntax (XHTML), on the other hand, all attributes must be quoted, and a trailing slash is required before the last angle bracket:

    <link rel="stylesheet" href="fancy.css" type="text/css" />

    Raw text elements are constructed with:

    Normal 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:

    HTML attributes define desired behaviour or indicate additional element properties. Most attributes require a value. In HTML, the value can be left unquoted if it doesn''t include spaces (name=value), or it can be quoted with single or double quotes (name=''value'' or name="value"). In XML, those quotes are required. Boolean attributes, on the other hand, don''t require a value to be specified. An example is the checked for checkboxes:

    <input type=checkbox checked>

    In the XML syntax, though, the name should be repeated as the value:

    <input type="checkbox" checked="checked" />

    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. 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.

    Element standards

    HTML elements are defined in a series of freely available open standards issued since 1995, initially by the IETF and subsequently by the W3C.

    Since the early 1990s, developers of user agents (e.g. web browsers) have often developed their own elements, some of which have been adopted in later standards. Other user agents may not recognize non-standard elements, and they may be ignored or displayed improperly.

    In 1998, XML (a simplified form of SGML) introduced mechanisms to allow anyone to develop their own elements and incorporate them in XHTML documents, for use with XML-aware user agents.

    Subsequently, HTML 4.01 was rewritten in an XML-compatible form, XHTML 1.0 (eXtensible HTML). The elements in each are identical, and in most cases valid XHTML 1.0 documents will be valid or nearly valid HTML 4.01 documents. This article mainly focuses on real HTML, unless noted otherwise; however, it remains applicable to XHTML. (See HTML for a discussion of the minor differences between the two).

    Element status

    Since the first version of HTML, several elements have become outmoded, and are deprecated in later standards, or do not appear at all, in which case they are invalid (and will be found invalid, and perhaps not displayed, by validating user agents).

    At present, the status of elements is complicated by the existence of three types of HTML 4.01 / XHTML 1.0 DTD:

    The first Standard (HTML 2.0) contained four deprecated elements, one of which was invalid in HTML 3.2. All four are invalid in HTML 4.01 Transitional, which also deprecated a further ten elements. All of these, plus two others, are invalid in HTML 4.01 Strict. While the frame elements are still current in the sense of being present in the Transitional and Frameset DTDs, there are no plans to preserve them in future standards, as their function has been largely replaced, and they are highly problematic for user accessibility.

    (Strictly speaking, the most recent XHTML standard, XHTML 1.1 (2001), does not include frames at all; it is approximately equivalent to XHTML 1.0 Strict, but also includes the Ruby markup module.)

    A common source of confusion is the loose use of deprecated to refer to both deprecated and invalid status, and to elements which are expected to be formally deprecated in future.

    Content vs. presentation and behavior

    Since HTML 4, HTML has increasingly focussed on the separation of content (the visible text and images) from presentation (like color, font size, and layout). This is often referred to as a separation of concerns. HTML is used to represent the structure or content of a document, its presentation remains the sole responsibility of CSS style sheets. A default style sheet is suggested as part of the CSS standard, giving a default rendering for HTML.

    Behavior (interactivity) is also kept separate from content, and is handled by scripts. Images are contained in separate graphics files, separate from text, though they can also be considered part of the content of a page.

    Separation of concerns allows the document to be presented by different user agents according to their purposes and abilities. For example, a user agent can select an appropriate style sheet to present a document by displaying on a monitor, printing on paper, or to determine speech characteristics in an audio-only user agent. The structural and semantic functions of the markup remain identical in each case.

    Historically, user agents did not always support these features. In the 1990s, as a stop-gap, presentational elements (like <b> and <i>) were added to HTML, at the cost of creating problems for interoperability and user accessibility. This is now regarded as outmoded and has been superseded by style sheet-based design; most presentational elements are now deprecated.

    External image files are incorporated with the img or object elements. (With XHTML, the SVG language can also be used to write graphics within the document, though linking to external SVG files is generally simpler.) Where an image is not purely decorative, HTML allows replacement content with similar semantic value to be provided for non-visual user agents.

    An HTML document can also be extended through the use of scripts to provide additional behaviours beyond the abilities of HTML hyperlinks and forms.

    The elements style and script, with related HTML attributes, provide reference points in HTML markup for links to style sheets and scripts. They can also contain instructions directly.

    Document structure elements

    <html>...</html>

    The Root element of an HTML document; all other elements are contained in this. The HTML element delimits the beginning and the end of an HTML document. Standardized in HTML 2.0; still current.

    <head>...</head>

    Container for processing information and metadata for an HTML document. Standardized in HTML 2.0; still current. (See Document head elements for child elements.)

    <body>...</body>

    Container for the displayable content of an HTML document. Standardized in HTML 2.0; still current. (See Document body elements for child elements.)Document head elements

    <base>

    Specifies a base URL for all relative href and other links in the document. Must appear before any element that refers to an external resource. HTML permits only one base element for each document. The base element has HTML attributes, but no contents. A development version of BASE is mentioned in HTML Tags; standardized in HTML 2.0; still current.

    <basefont> (deprecated)

    Specifies a base font size, typeface, and colour for the document. Used together with font elements. Deprecated in favour of style sheets. Standardized in HTML 3.2; deprecated in HTML 4.0 Transitional; invalid in HTML 4.0 Strict.

    <isindex> (deprecated)

    isindex could either appear in the document head or in the body, but only once in a document. See Forms.

    <link>

    Tags:Basic, Computer, Document Object Model, Dynamic, Dynamic HTML, HTML, HTML element, HTML5, Sydney, Web colors, Wikipedia, XHTML, XML


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