Well-formed element

In web page design, and generally for all

markup language Markup language 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 ...

s such as

SGML The Standard Generalized Markup Language (SGML; ISO 8879:1986) is a standard for defining generalized markup languages for documents. ISO 8879 Annex A.1 states that generalized markup is "based on two postulates": * Declarative: Markup should ...

HTML 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 JavaS ...

, and

XML 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. T ...

, 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 ''Nested'' is the seventh studio album by Bronx-born singer, songwriter and pianist Laura Nyro, released in 1978 on Columbia Records. Following on from her extensive tour to promote 1976's ''Smile'', which resulted in the 1977 live album '' Sea ...

so that it does not

overlap Overlap may refer to: * In set theory, an overlap of elements shared between sets is called an intersection, as in a Venn diagram. * In music theory, overlap is a synonym for reinterpretation of a chord at the boundary of two musical phrases * O ...

with other elements. For example, in HTML: word is a well-formed element, while is not, since the bold element is not closed. In

XHTML 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. While HTML, prior ...

, 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. etc. In HTML 4.01 and earlier, no slash is added to terminate the element.

HTML5 HTML5 is a markup language used for structuring and presenting content on the World Wide Web. It is the fifth and final major HTML version that is a World Wide Web Consortium (W3C) recommendation. The current specification is known as the HTML ...

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 
Strong may refer to:

 Education
* The Strong, an educational institution in Rochester, New York, United States
*  Strong Hall (Lawrence, Kansas), an administrative hall of the University of Kansas
*  Strong School, New Haven, Connecticut, United S ...

elements overlap:

Normal emphasized strong emphasized strong

Alternatively emphasized strong emphasized strong

, 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 instruction A Processing Instruction (PI) is an SGML and XML node type, which may occur anywhere in the document, intended to carry instructions to the application. Processing instructions are exposed in the Document Object Model as Node.PROCESSING_INSTRUCTION ...

s, 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 ( GML, SGML, XML, HTML). A DTD defines the valid building blocks of an XML document. It defines the document s ...

). XML HTML {{Web-stub