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. Many 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
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 cannot contain any content or other elements. For example, the address of the dentist in the movie Finding Nemo would be written as
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).
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
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" 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:
can thus be inferred to be equivalent to:
(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.
Comment Box In Html Video
Overview
Syntax
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
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:
Raw text elements are constructed with:
- a start tag (
<tag>) 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:
</tag>. In some versions of HTML, the end tag is optional for some elements. The end tag is required in XHTML.
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:
- a start tag (
<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:
</tag>.
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:
In the XML syntax, though, the name should be repeated as the value:
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:
- Transitional, which contain deprecated elements, but which were intended to provide a transitional period during which authors could update their practices;
- Frameset, which are versions of the Transitional DTDs which also allow authors to write frameset documents;
- Strict, which is the up-to date (as at 1999) form of HTML.
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 focused 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.
- In the document head, script and style may either link to shared external documents, or contain embedded instructions. (The link element can also be used to link style sheets.)
- The style attribute is valid in most document body elements for inclusion of inline style instructions.
- Event-handling attributes, which provide links to scripts, are optional in most elements.
- script can occur at any point in the document body.
- For user agents which do not operate scripts, the noscript element provides alternative content where appropriate; however, it can only be used as a block-level element.
Document structure elements
<html>...</html>
<head>...</head>
<body>...</body>
Document head elements
<base>
<basefont> (deprecated)
<isindex> (deprecated)
<link>
head element may contain any number of link elements. The link element has HTML attributes, but no contents.<meta>
foo that has a value bar.meta element specifies name and associated content HTML attributes describing aspects of the HTML page. To prevent possible ambiguity, an optional third attribute, scheme, may be supplied to specify a semantic framework that defines the meaning of the key and its value: for example: meta element identifies itself as containing the foo element, with a value of bar, from the DC or Dublin Core resource description framework.<object>...</object>
<script>...</script>
<style>...</style>
@import directives of the form: <title>...</title>
Document body elements
In visual browsers, displayable elements can be rendered as either block or inline. While all elements are part of the document sequence, block elements appear within their parent elements:
- as rectangular objects which do not break across lines;
- with block margins, width and height properties which can be set independently of the surrounding elements.
Conversely, inline elements are treated as part of the flow of document text; they cannot have margins, width or height set, and do break across lines.
Block elements
Block elements, or block-level elements, have a rectangular structure. By default, these elements will span the entire width of its parent element, and will thus not allow any other element to occupy the same horizontal space as it is placed on.
The rectangular structure of a block element is often referred to as the box model, and is made up of several parts. Each element contains the following:
- The content of an element is the actual text (or other media) placed between the opening and closing tags of an element.
- The padding of an element is the space around that content, which still form part of said element. Padding is physically part of an element, and should not be used to create white space between two elements. Any background style assigned to the element, such as a background image or color, will be visible within the padding. Increasing the size of an element's padding increases the space this element will take up.
- The border of an element is the absolute end of an element, and spans the perimeter of that element. The thickness of a border increases the size of an element.
- The margin of an element is the white-space that surrounds an element. The content, padding and border of any other element will not be allowed to enter this area, unless forced to do so by some advanced CSS placement. Using most standard DTDs, margins on the left and right of different elements will push each other away. Margins on the top or bottom of an element, on the other hand, will not stack, or will inter mingle. This means that the white-space between these elements will be as big as the larger margin between them.
The above section refers only to the detailed implementation of CSS rendering and has no relevance to HTML elements themselves.
Basic text
<p>...</p>
<h1>...</h1> <h2>...</h2> <h3>...</h3> <h4>...</h4> <h5>...</h5> <h6>...</h6>
Lists
<dl>...</dl>
<dt>...</dt>
<dd>...</dd>
<ol>...</ol>
<ul>...</ul>
<li>...</li>
<dir>...</dir> (deprecated)
Other block elements
<address>...</address>
<blockquote>...</blockquote>
<center>...</center>
<del>...</del>
<div>...</div>
<hr>
<ins>...</ins>
<noscript>...</noscript>
<pre>...</pre>
<script>...</script>
Inline elements
Inline elements cannot be placed directly inside the body element; they must be wholly nested within block-level elements.
Anchor
<a>...</a>
title is displayed in a tooltip or in some other manner. Some browsers render alt text the same way, despite this not being what the specification calls for.A existed in HTML Tags, and was standardized in HTML 2.0; still current.Phrase elements
General
<abbr>...</abbr>
<acronym>...</acronym> (deprecated)
<dfn>...</dfn>
<em>...</em>
<strong>...</strong>
Computer phrase elements
These elements are useful primarily for documenting computer code development and user interaction through differentiation of source code (<code>), source code variables (<var>), user input (<kbd>), and terminal output (<samp>).
<code>...</code>
<samp>...</samp>
<kbd>...</kbd>
<var>...</var>
Presentation
As visual presentational markup only applies directly to visual browsers, its use is discouraged. Style sheets should be used instead. Several of these elements are deprecated or invalid in HTML 4 / XHTML 1.0, and the remainder are invalid in the current draft of XHTML 2.0. The current draft of HTML 5, however, re-includes <s>, <u>, and <small>, assigning new semantic meaning to each. In an HTML 5 document, the use of these elements is no longer discouraged, provided that it is semantically correct.
<b>...</b>
<i>...</i>
<u>...</u>
<small>...</small>
<s>...</s>
<big>...</big>
<strike>...</strike>
<tt>...</tt>
<font>...</font>
Span
<span>...</span>
Other inline elements
<br>
<bdo>...</bdo>
<cite>...</cite>
<del>...</del>
<ins>...</ins>
<q>...</q>
<script>...</script>
<sub>...</sub> and <sup>...</sup>
<wbr>
Images and objects
<applet>...</applet> (deprecated)
<area>
<img>
<map>...</map>
<object>...</object>
<param>
Forms
These elements can be combined into a form or in some instances used separately as user-interface controls; in the document, they can be simple HTML or used in conjunction with Scripts. HTML markup specifies the elements that make up a form, and the method by which it will be submitted. However, some form of scripts (server-side, client-side, or both) must be used to process the user's input once it is submitted.
(These elements are either block or inline elements, but are collected here as their use is more restricted than other inline or block elements.)
<form action="url">...</form>
<button>...</button>
<fieldset>...</fieldset>
<input>
<isindex> (deprecated)
<label for="id">...</label>
<legend>...</legend>
<option value="x">
<optgroup>...</optgroup>
<select name="xyz">...</select>
<textarea rows="8">...</textarea>
Tables
The format of HTML Tables was proposed in the HTML 3.0 Drafts and the later RFC 1942 HTML Tables. They were inspired by the CALS Table Model. Some elements in these proposals were included in HTML 3.2; the present form of HTML Tables was standardized in HTML 4. (Many of the elements used within tables are neither block nor inline elements.)
Frames
Frames allow a visual HTML Browser window to be split into segments, each of which can show a different document. This can lower bandwidth use, as repeating parts of a layout can be used in one frame, while variable content is displayed in another. This may come at a certain usability cost, especially in non-visual user agents, due to separate and independent documents (or websites) being displayed adjacent to each other and being allowed to interact with the same parent window. Because of this cost, frames (excluding the <iframe> element) are only allowed in HTML 4.01 Frame-set. Iframes can also hold documents on different servers. In this case the interaction between windows is blocked by the browser. Sites like Facebook and twitter use iframes to display content (plugins) on third party websites. Google Adsense uses iframes to display banners on third party websites.
In HTML 4.01, a document may contain a <head> and a <body> or a <head> and a <frameset>, but not both a <body> and a <frameset>. However, <iframe> can be used in a normal document body.
Longdesc
In HTML, longdesc is an attribute used within the image element, frame element, or iframe element. It is supposed to be a URL to a document that provides a long description for the image, frame, or iframe in question. Note that this attribute should contain a URL, and not as is commonly mistaken, the text of the description itself.
Longdesc was designed to be used by screen readers to display image information for computer users with accessibility issues, such as the blind or visually impaired, and is widely implemented by both web browsers and screen readers. Some developers object that it is actually seldom used for this purpose, because there are relatively few authors who use the attribute, and most of those authors use it incorrectly, and have used this argument to recommend dropping longdesc. The publishing industry has responded, advocating the retention of longdesc.
Example
Content of description.html:
Linking to the long description in the text
Since very few graphical browsers support making the link available natively (Opera and iCab being the exceptions), it is useful to include a link to the description page near the img element whenever possible, as this can also aid sighted users.
Example
Historic elements
The following elements were part of the early HTML developed by Tim Berners-Lee from 1989-91; they are mentioned in HTML Tags, but deprecated in HTML 2.0 and were never part of HTML standards.
Non-standard elements
This section lists some widely used obsolete elements, which means they are not used in valid code. They may not be supported in all user agents.
Previously obsolete but added back in HTML 5
<menu>...</menu>
Comments
A comment in HTML (and related XML, SGML and SHTML) uses the same syntax as the SGML comment or XML comment, depending on the doctype.
Unlike most HTML tags, comments do not nest.
The markup <!--Xbegin<!--Y-->Xend--> will yield the comment Xbegin<!--Y and the text Xend--> after it.
Comments can appear anywhere in a document, as the HTML parser is supposed to ignore them no matter where they appear so long as they are not inside other HTML tag structures.
Comments can even appear before the doctype declaration; no other tags are permitted to do this.
However, not all browsers and HTML editors are fully compliant with the HTML syntax framework and may do unpredictable things under some syntax conditions. Defective handling of comments only affects about 5% of all browsers and HTML editors in use (IE6 accounting for most of this high percentage). Even then only certain versions are affected by comment mishandling issues.
There are a few compatibility quirks involving comments:
- Placing comments - or indeed any characters except for white-space - before the doctype will cause Internet Explorer 6 to use quirks mode for the HTML page. None of its enclosed contents are processed.
- For compatibility with some pre-1995 browsers, the contents of
styleandscriptelements are still sometimes surrounded by comment delimiters. - The BlueGriffon HTML editor, in versions 1.7.x makes comments that are not embedded in the syntax structure
<style> ... {comment tags} ...</style>show up on screen. Other HTML editors may have this same defect.
Notes
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