In web page design, and generally for all markup languages such as SGML, HTML, and XML, a well-formed element is one that is either a) opened and subsequently closed, or b) an empty element, which in that case must be terminated; and in either case which is properly nested so that it does not overlap with other elements.
For example, in HTML: <b>word</b> is a well-formed element, while <i><b>word</i> is not, since the bold element <b> is not closed.
In XHTML, and XML, empty elements (elements that inherently have no content) are terminated by putting a slash at the end of the "opening" (only) tag, e.g. <img/>,<br/>,<hr/>, etc. In HTML 4.01 and earlier, no slash is added to terminate the element. HTML5 does not require one, but it is often added for compatibility with XHTML and XML processing.
In a well-formed document,
all elements are well-formed, and
a single element, known as the root element, contains all of the other elements in the document.
For example, the code below is not well-formed HTML, because the em and strong elements overlap:
<!-- WRONG! NOT well-formed HTML! --><p>Normal <em>emphasized <strong>strong emphasized</em> strong</strong></p>
In XML, the phrase well-formed document is often used to describe a text that follows all the syntactic rules as well-formedness rules in the XML specification: strictly speaking the phrase is tautological, since a text that does not follow these rules is not an XML document. The rules for well-formed XML documents go beyond the general requirements for the markup languages mentioned above. The additional rules include, for example, a rule to quote attribute values, case-sensitiveness of tag names, rules about the characters that can appear in names and elsewhere, the syntax of comments, processing instructions, entity references, and CDATA sections, and many other similar details. Sometimes the adjective well-formed is used to contrast with valid: a valid XML document is one that is not only well-formed, but also conforms to the grammar defined in its own DTD (Document Type Definition).
A document type definition (DTD) is a set of markup declarations that define a document type for an SGML-family markup language.
The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaScript.
Markuplanguage refers to a text-encoding system consisting of a set of symbols inserted in a text document to control its structure, formatting, or the relationship between its parts. Markup is often used to control the display of the document or to enrich its content to facilitating automated processing. A markup language is a set of rules governing what markup information may be included in a document and how it is combined with the content of the document in a way to facilitate use by humans and computer programs. The idea and terminology evolved from the "marking up" of paper manuscripts, which is traditionally written with a red pen or blue pencil on authors' manuscripts.
The Standard Generalized Markup Language is a standard for defining generalized markup languages for documents. ISO 8879 Annex A.1 states that generalized markup is "based on two postulates":
Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. The World Wide Web Consortium's XML 1.0 Specification of 1998 and several other related specifications—all of them free open standards—define XML.
Mathematical Markup Language (MathML) is a mathematical markup language, an application of XML for describing mathematical notations and capturing both its structure and content. It aims at integrating mathematical formulae into World Wide Web pages and other documents. It is part of HTML5 and is a ISO/IEC standard ISO/IEC 40314 since 2015.
An HTML element is a type of HTML document component, one of several types of HTML nodes. The first used version of HTML was written by Tim Berners-Lee in 1993 and there have since been many versions of HTML. The most commonly used version is HTML 4.01, which became official standard in December 1999. An HTML document is composed of a tree of simple HTML nodes, such as text nodes, and HTML elements, which add semantics and formatting to parts of document. Each element can have HTML attributes specified. Elements can also have content, including other elements and text.
In computing, a polyglot is a computer program or script written in a valid form of multiple programming languages or file formats. The name was coined by analogy to multilingualism. A polyglot file is composed by combining syntax from two or more different formats. When the file formats are to be compiled or interpreted as source code, the file can be said to be a polyglot program, though file formats and source code syntax are both fundamentally streams of bytes, and exploiting this commonality is key to the development of polyglots. Polyglot files have practical applications in compatibility, but can also present a security risk when used to bypass validation or to exploit a vulnerability.
In web development, "tag soup" is a pejorative for syntactically or structurally incorrect HTML written for a web page. Because web browsers have historically treated structural or syntax errors in HTML leniently, there has been little pressure for web developers to follow published standards, and therefore there is a need for all browser implementations to provide mechanisms to cope with the appearance of "tag soup", accepting and correcting for invalid syntax and structure where possible.
The term CDATA, meaning character data, is used for distinct, but related, purposes in the markup languages SGML and XML. The term indicates that a certain portion of the document is general character data, rather than non-character data or character data with a more specific, limited structure.
In HTML, div and span tags are elements used to define parts of a document, so that they are identifiable when a unique classification is necessary. Where other HTML elements such as p (paragraph), em (emphasis), and so on, accurately represent the semantics of the content, the additional use of span and div tags leads to better accessibility for readers and easier maintainability for authors. Where no existing HTML element is applicable, span and div can valuably represent parts of a document so that HTML attributes such as class, id, lang, or dir can be applied.
OmniMark is a fourth-generation programming language used mostly in the publishing industry. It is currently a proprietary software product of Stilo International. As of July 2022, the most recent release of OmniMark was 11.0.
RDFa or Resource Description Framework in Attributes is a W3C Recommendation that adds a set of attribute-level extensions to HTML, XHTML and various XML-based document types for embedding rich metadata within Web documents. The Resource Description Framework (RDF) data-model mapping enables its use for embedding RDF subject-predicate-object expressions within XHTML documents. It also enables the extraction of RDF model triples by compliant user agents.
Extensible HyperText Markup Language (XHTML) is part of the family of XML markup languages. It mirrors or extends versions of the widely used HyperText Markup Language (HTML), the language in which Web pages are formulated.
A structured document is an electronic document where some method of markup is used to identify the whole and parts of the document as having various meanings beyond their formatting. For example, a structured document might identify a certain portion as a "chapter title" rather than as "Helvetica bold 24" or "indented Courier". Such portions in general are commonly called "components" or "elements" of a document.
HTML attributes are special words used inside the opening tag to control the element's behaviour. HTML attributes are a modifier of an HTML element type. An attribute either modifies the default functionality of an element type or provides functionality to certain element types unable to function correctly without them. In HTML syntax, an attribute is added to an HTML start tag.
XHTML+RDFa is an extended version of the XHTML markup language for supporting RDF through a collection of attributes and processing rules in the form of well-formed XML documents. XHTML+RDFa is one of the techniques used to develop Semantic Web content by embedding rich semantic markup. Version 1.1 of the language is a superset of XHTML 1.1, integrating the attributes according to RDFa Core 1.1. In other words, it is an RDFa support through XHTML Modularization.
A well-formed document in XML is a document that "adheres to the syntax rules specified by the XML 1.0 specification in that it must satisfy both physical and logical structures".
In computing, a polyglot markup is a document or script written in a valid form of multiple markup languages, which performs the same output, independent of the markup's parser, layout engine, or interpreter. In general, the polyglot markup is a common subset of two or more languages, that can be used as a robust or simplified profile.
A document type declaration, or DOCTYPE, is an instruction that associates a particular XML or SGML document with a document type definition (DTD). In the serialized form of the document, it manifests as a short string of markup that conforms to a particular syntax.
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