HTML element
[1]Similarly the W3C Recommendation HTML 5.1 2nd Edition explicitly says: Tags are used to delimit the start and end of elements in the markup.
Although both can use the DTD to specify the supported elements and their permitted combinations as document structure, XML parsing is simpler.
[note 2] 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.
For example, the element, which represents an abbreviation, expects a title attribute within its opening tag.
Two common void elements are
(for a hard line-break, such as in a poem or an address) and
(for a thematic break).
In the HTML syntax attributes do not have to be quoted if they are composed only of certain characters: letters, digits, the hyphen-minus and the period.
When using the XML syntax (XHTML), on the other hand, all attributes must be quoted, and a spaced trailing slash is required before the last angle bracket: HTML attributes define desired behavior or indicate additional element properties.
An example is the checked for checkboxes: In the XML (and thus XHTML) syntax, though, a value is required, and the name should be repeated as the value: HTML elements are defined in a series of freely available open standards issued since 1995, initially by the IETF and subsequently by the W3C.
Other user agents may not recognize non-standard elements, and they will be ignored, possibly causing the page to be 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.
[10] In HTML 4.01 / XHTML 1.0, the status of elements is complicated by the existence of three types of DTD: HTML5 instead provides a listing of obsolete features to go along with the standardized normative content.
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.
HTML is used to represent the structure or content of a document, its presentation remains the sole responsibility of CSS style sheets.
Separation of concerns allows the document to be presented by different user agents according to their purposes and abilities.
In the 1990s, as a stop-gap, presentational elements (like and ) 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.
)[16] Where an image is not purely decorative, HTML allows replacement content with similar semantic value to be provided for non-visual user agents.
)Can be used to specify additional metadata about a document, such as its author, publication date, expiration date, language, page title, page description, keywords, or other information not provided through the other header elements and HTML attributes.
In one form, elements can specify HTTP headers which should be sent by a web server before the actual content.
The rectangular structure of a block element is often referred to as the box model, and is made up of several parts.
As an origin, setting the attribute href,[26] creates a hyperlink; it can point to either another part of the document or another resource (e.g. a webpage) using an external URL.
As a target, setting the name or id HTML attributes, allows the element to be linked from a Uniform Resource Locator (URL) via a fragment identifier.
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.
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,[54] due to separate and independent documents (or websites) being displayed adjacent to each other and being allowed to interact with the same parent window.
Sites like Facebook and Twitter use iframes to display content (plugins) on third party websites.
It is supposed to be a URL[note 5] to a document that provides a long description for the image, frame, or iframe in question.
UuU This section lists some widely used obsolete elements, which means they are not used in valid code.
