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Based of HTML

HTML as a hypertext format

HTML is the basis of a comparatively weak hypertext implementation. Earlier hypertext systems had features such as typed links, transclusion, and source tracking. Another feature lacking today is fat links.

Even some hypertext features that were in early versions of HTML have been ignored by most popular web browsers until recently, such as the link element and editable web pages.

Sometimes web services or browser manufacturers remedy these shortcomings. For instance, members of the modern social software landscape such as wikis and content management systems allow surfers to edit the web pages they visit.

HTML markup

HTML markup consists of several types of entities, including: elements, attributes, data types and character references.

The Document Type Definition

In order to enable Document Type Definition (DTD)-based validation with SGML tools and in order to avoid the Quirks mode in browsers, all HTML documents should start with a Document Type Declaration (informally, a "DOCTYPE"). The DTD contains machine readable grammar specifying the permitted and prohibited content for a document conforming to such a DTD. Browsers do not direct to the page in the DTD, however. Browsers only look at the doctype in order to decide the layout mode. Not all doctypes trigger the Standards layout mode avoiding the Quirks mode. For example:

This declaration references the Strict DTD of HTML 4.01, which does not have presentational elements like , leaving formatting to Cascading Style Sheets and the span and div tags. SGML-based validators read the DTD in order to properly parse the document and to perform validation. In modern browsers, the HTML 4.01 Strict doctype activates the Standards layout mode for CSS as opposed to the Quirks mode.

In addition, HTML 4.01 provides Transitional and Frameset DTDs. The Transitional DTD was intended to gradually phase in the changes made in the Strict DTD, while the Frameset DTD was intended for those documents which contained frames.

Elements

See HTML elements for more detailed descriptions.

Elements are the basic structure for HTML markup. Elements have two basic properties: attributes and content. Each attribute and each element's content has certain restrictions that must be followed for an HTML document to be considered valid. An element usually has a start label (e.g. ) and an end label (e.g. ). The element's attributes are contained in the start label and content is located between the labels (e.g. Content). Some elements, such as
, will never have any content and do not need closing labels. Listed below are several types of markup elements used in HTML.

Structural markup describes the purpose of text. For example,

Golf

establishes "Golf" as a second-level heading, which would be rendered in a browser in a manner similar to the "HTML markup" title at the start of this section. A blank line is included after the header. Structural markup does not denote any specific rendering, but most web browsers have standardized on how elements should be formatted. Further styling should be done with Cascading Style Sheets (CSS).

Presentational markup describes the appearance of the text, regardless of its function. For example boldface indicates that visual output devices should render "boldface" in bold text, but has no clear semantics for aural devices that read the text aloud for the sight-impaired. In the case of both bold and italic there are elements which usually have an equivalent visual rendering but are more semantic in nature, namely strong emphasis and emphasis respectively. It is easier to see how an aural user agent should interpret the latter two elements. However, they are not equivalent to their presentational counterparts: it would be undesirable for a screen-reader to emphasize the name of a book, for instance, but on a screen such a name would be italicized. Most presentational markup elements have become deprecated under the HTML 4.0 specification, in favor of CSS based style design.

Hypertext markup links parts of the document to other documents. HTML up through version XHTML 1.1 requires the use of an anchor element to create a hyperlink in the flow of text: Wikipedia. However, the href attribute must also be set to a valid URL so for example the HTML code, Wikipedia, will render the word "Wikipedia" as a hyperlink.

Attributes

The attributes of an element are name-value pairs, separated by "=", and written within the start label of an element, after the element's name. The value should be enclosed in single or double quotes, although values consisting of certain characters can be left unquoted in HTML (but not XHTML).Leaving attribute values unquoted is considered unsafe.

Most elements take any of several common attributes: id, class, style and title. Most also take language-related attributes: lang and dir.

The id attribute provides a document-wide unique identifier for an element. This can be used by stylesheets to provide presentational properties, by browsers to focus attention on the specific element or by scripts to alter the contents or presentation of an element. The class attribute provides a way of classifying similar elements for presentation purposes. For example, an HTML (or a set of documents) document may use the designation class="notation" to indicate that all elements with this class value are all subordinate to the main text of the document (or documents). Such notation classes of elements might be gathered together and presented as footnotes on a page, rather than appearing in the place where they appear in the source HTML.

An author may use the style non-attributal codes presentational properties to a particular element. It is considered better practice to use an element’s son- id page and select the element with a stylesheet, though sometimes this can be too cumbersome for a simple ad hoc application of styled properties. The title is used to attach subtextual explanation to an element. In most browsers this title attribute is displayed as what is often referred to as a tooltip. The generic inline span element can be used to demonstrate these various non-attributes.

HTML

which displays as HTML (pointing the cursor at the abbreviation should display the title text in most browsers).

Other markup

As of version 4.0, HTML defines a set of 252 character entity references and a set of 1,114,050 numeric character references, both of which allow individual characters to be written via simple markup, rather than literally. A literal character and its markup equivalent are considered equivalent and are rendered identically.

The ability to "escape" characters in this way allows for the characters "<" and "&" (when written as < and &, respectively) to be interpreted as character data, rather than markup. For example, a literal "<" normally indicates the start of a label, and "&" normally indicates the start of a character entity reference or numeric character reference; writing it as "&" or "&" allows "&" to be included in the content of elements or the values of attributes. The double-quote character, ", when used to quote an attribute value, must also be escaped as """ or "" when it appears within in the attribute value itself. However, since document authors often overlook the need to escape these characters, browsers tend to be very forgiving, treating them as markup only when subsequent text appears to confirm that intent.

Escaping also allows for characters that are not easily typed or that aren't even available in the document's character encoding to be represented within the element and attribute content. For example, "é", a character typically found only on Western European keyboards, can be written in any HTML document as the entity reference é or as the numeric references é or é. The characters comprising those references (that is, the "&", the ";", the letters in "eacute", and so on) are available on all keyboards and are supported in all character encodings, whereas the literal "é" is not.

HTML also defines several data types for element content, such as script data and stylesheet data, and a plethora of types for attribute values, including IDs, names, URIs, numbers, units of length, languages, media descriptors, colors, character encodings, dates and times, and so on. All of these data types are specializations of character data.

Semantic HTML

There is no official specification called "Semantic HTML", though the strict flavors of HTML discussed below are a push in that direction. Rather, semantic HTML refers to an objective and a practice to create documents with HTML that contain only the author's intended meaning, without any reference to how this meaning is presented or conveyed. A classic example is the distinction between the emphasis element () and the italics element (). Often the emphasis element is displayed in italics, so the presentation is typically the same. However, emphasizing something is different from listing the title of a book, for example, which may also be displayed in italics. In purely semantic HTML, a book title would use a separate element than emphasized text uses (for example a ), because they are each meaningfully different things.

The goal of semantic HTML requires two things of authors:

1) to avoid the use of presentational markup (elements, attributes and other entities); 2) the use of available markup to differentiate the meanings of phrases and structure in the document. So for example, the book title from above would need to have its own element and class specified such as The Grapes of Wrath. Here, the element is used, because it most closely matches the meaning of this phrase in the text. However, the element is not specific enough to this task because we mean to cite specifically a book title as opposed to a newspaper article or a particular academic journal.

Semantic HTML also requires complementary specifications and software compliance with these specifications. Primarily, the development and proliferation of CSS has led to increasing support for semantic HTML because CSS provides designers with a rich language to alter the presentation of semantic-only documents. With the development of CSS the need to include presentational properties in a document has virtually disappeared. With the advent and refinement of CSS and the increasing support for it in web browsers, subsequent editions of HTML increasingly stress only using markup that suggests the semantic structure and phrasing of the document, like headings, paragraphs, quotes, and lists, instead of using markup which is written for visual purposes only, like , (bold), and (italics). Some of these elements are not permitted in certain varieties of HTML, like HTML 4.01 Strict. CSS provides a way to separate document semantics from the content's presentation, by keeping everything relevant to presentation defined in a CSS file. See separation of style and content.

Semantic HTML offers many advantages. First, it ensures consistency in style across elements that have the same meaning. Every heading, every quotation mark, every similar element receives the same presentation properties.

Second, semantic HTML frees authors from the need to concern themselves with presentation details. When writing the number two, for example, should it be written out in words ("two"), or should it be written as a numeral (2)? A semantic markup might enter something like 2 and leave presentation details to the stylesheet designers. Similarly, an author might wonder where to break out quotations into separate indented blocks of text - with purely semantic HTML, such details would be left up to stylesheet designers. Authors would simply indicate quotations when they occur in the text, and not concern themselves with presentation.

A third advantage is device independence and repurposing of documents. A semantic HTML document can be paired with any number of stylesheets to provide output to computer screens (through web browsers), high-resolution printers, handheld devices, aural browsers or braille devices for those with visual impairments, and so on. To accomplish this nothing needs to be changed in a well coded semantic HTML document. Readily available stylesheets make this a simple matter of pairing a semantic HTML document with the appropriate stylesheets (of course, the stylesheet's selectors need to match the appropriate properties in the HTML document).

Some aspects of authoring documents make separating semantics from style (in other words, meaning from presentation) difficult. Some elements are hybrids, using presentation in their very meaning. For example, a table displays content in a tabular form. Often this content only conveys the meaning when presented in this way. Repurposing a table for an aural device typically involves somehow presenting the table as an inherently visual element in an audible form. On the other hand, we frequently present lyrical songs — something inherently meant for audible presentation — and instead present them in textual form on a web page. For these types of elements, the meaning is not so easily separated from their presentation. However, for a great many of the elements used and meanings conveyed in HTML the translation is relatively smooth.

Delivery of HTML

HTML documents can be delivered by the same means as any other computer file; however, HTML documents are most often delivered in one of the following two forms: Over HTTPservers and through email.

Refference :