Markup
HTML markup consists of several key components, including tags (and their attributes), character-based data types, character references and entity references. Another important component is the document type declaration, which triggers standards mode rendering.
The following is an example of the classic Hello world program, a common test employed for comparing programming languages, scripting languages and markup languages. This example is made using 9 lines of code:
<!DOCTYPE html> <html> <head> <title>This is a title</title> </head> <body> <p>Hello world!</p> </body> </html>
(The text between <html> and </html> describes the web page, and the text between <body> and </body> is the visible page content. The markup text "<title>This is a title</title>" defines the browser page title.)
This Document Type Declaration is for HTML5. If the
<!DOCTYPE html> declaration is not included, various browsers will revert to "quirks mode" for renderingElements
HTML documents imply a structure of nested HTML elements. These are indicated in the document by HTML tags, enclosed in angle brackets thus:
<p>
In the simple, general case, the extent of an element is indicated by a pair of tags: a "start tag"
<p> and "end tag" </p>. The text content of the element, if any, is placed between these tags.
Tags may also enclose further tag markup between the start and end, including a mixture of tags and text. This indicates further, nested, elements, as children of the parent element.
Character and entity references
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 counterpart 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 tag, and & normally indicates the start of a character entity reference or numeric character reference; writing it as & or & or & allows & to be included in the content of an element or in the value of an attribute. The double-quote character ("), when not used to quote an attribute value, must also be escaped as " or " or " when it appears within the attribute value itself. Equivalently, the single-quote character ('), when not used to quote an attribute value, must also be escaped as ' or ' (or as ' in HTML5 or XHTML documents ) when it appears within the attribute value itself. If document authors overlook the need to escape such characters, some browsers can be very forgiving and try to use context to guess their intent. The result is still invalid markup, which makes the document less accessible to other browsers and to other user agents that may try to parse the document for search and indexing
purposes for example.
Data types
HTML 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.
Document type declaration
HTML documents are required to start with a Document Type Declaration (informally, a "doctype"). In browsers, the doctype helps to define the rendering mode—particularly whether to use quirks mode.
HTML5 does not define a DTD; therefore, in HTML5 the doctype declaration is simpler and shorter:
<!DOCTYPE html>
An example of an HTML 4 doctype
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
HTML4 variations
Since its inception, HTML and its associated protocols gained acceptance relatively quickly. However, no clear standards existed in the early years of the language. Though its creators originally conceived of HTML as a semantic language devoid of presentation details,[65]practical uses pushed many presentational elements and attributes into the language, driven largely by the various browser vendors
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