Document type declaration
A document type declaration, or DOCTYPE, is an instruction that associates a particular
The HTML layout engines in modern web browsers perform DOCTYPE "sniffing" or "switching", wherein the DOCTYPE in a document served as text/html
determines a layout mode, such as "quirks mode" or "standards mode". The text/html
serialization of HTML5, which is not SGML-based, uses the DOCTYPE only for mode selection. Since web browsers are implemented with special-purpose HTML parsers, rather than general-purpose DTD-based parsers, they do not use DTDs and never access them even if a URL is provided. The DOCTYPE is retained in HTML5 as a "mostly useless, but required" header only to trigger "standards mode" in common browsers.[2]
Syntax
The general syntax for a document type declaration is:
<!DOCTYPE root-element PUBLIC "/quotedFPI/" "/quotedURI/" [
<!-- internal subset declarations -->
]>
or
<!DOCTYPE root-element SYSTEM "/quotedURI/" [
<!-- internal subset declarations -->
]>
Document type name
The opening <!DOCTYPE
syntax is followed by separating syntax[3]: 403–404 (such as spaces,[3]: 297–298, 372 or (except in XML) comments opened and closed by a doubled ASCII hyphen),[3]: 372, 391 followed by a document type name[3]: 403–404 (i.e. the name of the root element that the DTD applies to trees descending from). In XML, the root element that represents the document is the first element in the document. For example, in XHTML, the root element is <html>, being the first element opened (after the doctype declaration) and last closed.
Since the syntax for the external identifier and internal subset are both optional,[3]: 403–404 the document type name is the only information which it is mandatory to give in a DOCTYPE declaration.
External identifier
The DOCTYPE declaration can optionally contain an
The public identifier (if present) or SYSTEM
keyword (otherwise) may (and, in XML, must)"-//W3C//DTD XHTML 1.1//EN"
and, there are 3 possible system identifiers available for XHTML 1.1 depending on the needs. One of them is the URL"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
. It means that the XML parser must locate the DTD in a system specific fashion, in this case, by means of a URL reference of the DTD enclosed in double quote marks.
In XHTML documents, the doctype declaration must always explicitly specify a system identifier. In SGML-based documents like HTML, on the other hand, the appropriate system identifier may automatically be inferred from the given public identifier. This association might e.g. be performed by means of a catalog file resolving the FPI to a system identifier.[6] The SYSTEM
keyword can (except in XML) also be used without a system identifier following, indicating that a DTD exists but should be inferred from the document type name.[3]: 378
Internal subset
The last, optional, part of a DOCTYPE declaration is surrounded by literal square brackets ([]
), and called an internal subset. It can be used to add/edit entities or add/edit PUBLIC keyword behaviors.[7] It is possible, but uncommon, to include the entire DTD in-line in the document, within the internal subset, rather than referencing it from an external file.[3]: 402 Conversely, the internal subset is sometimes forbidden within simple SGML profiles, notably those for basic HTML parsers that don't implement a full SGML parser.
If both an internal DTD subset and an external identifier are included in a DOCTYPE declaration, the internal subset is processed first, and the external DTD subset is treated as if it were transcluded at the end of the internal subset. Since earlier definitions take precedence over later definitions in a DTD, this allows the internal subset to override definitions in the external subset.[3]: 402–403
Example
The first line of a World Wide Web page may read as follows:
<!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html lang="ar" dir="ltr" xmlns="http://www.w3.org/1999/xhtml">
This document type declaration for XHTML includes by reference a DTD, whose public identifier is -//W3C//DTD XHTML 1.0 Transitional//EN
and whose system identifier is http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd
. An entity resolver may use either identifier for locating the referenced external entity. No internal subset has been indicated in this example or the next ones. The root element is declared to be html
and, therefore, it is the first tag to be opened after the end of the doctype declaration in this example and the next ones, too. The HTML tag is not part of the doctype declaration but has been included in the examples for orientation purposes.
Common DTDs
Some common DTDs have been put into lists. W3C has produced a list of DTDs commonly used in the web, which contains the "bare" HTML5 DTD, older XHTML/HTML DTDs, DTDs of common embedded XML-based formats like
HTML 4.01 DTDs
Strict DTD does not allow presentational markup with the argument that Cascading Style Sheets should be used for that instead. This is how the Strict DTD looks:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
Transitional DTD allows some older PUBLIC and attributes that have been deprecated:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
If
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN"
"http://www.w3.org/TR/html4/frameset.dtd">
<html>
XHTML 1.0 DTDs
XHTML's DTDs are also Strict, Transitional and Frameset.
XHTML Strict DTD. No
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
XHTML Transitional DTD is like the XHTML Strict DTD, but deprecated tags are allowed.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
XHTML Frameset DTD is the only XHTML DTD that supports Frameset. The DTD is below.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
XHTML 1.1 DTD
XHTML 1.1 is the most current finalized revision of XHTML, introducing support for XHTML Modularization. XHTML 1.1 has the stringency of XHTML 1.0 Strict.
<!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
XHTML Basic DTDs
XHTML Basic 1.0
<!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML Basic 1.0//EN"
"http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
XHTML Basic 1.1
<!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML Basic 1.1//EN"
"http://www.w3.org/TR/xhtml-basic/xhtml-basic11.dtd">
HTML5 DTD-less DOCTYPE
HTML5 uses a DOCTYPE
declaration which is very short, due to its lack of references to a DTD in the form of a URL or FPI. All it contains is the tag name of the root element of the document, HTML
.[10] In the words of the specification draft itself:
<!DOCTYPE html>
, case-insensitively.
With the exception of the lack of a URI or the FPI string (the FPI string is treated case sensitively by validators), this format (a case-insensitive match of the string !DOCTYPE HTML
) is the same as found in the syntax of the SGML based HTML 4.01 DOCTYPE
. Both in HTML4 and in HTML5, the formal syntax is defined in upper case letters, even if both lower case and mixes of lower case upper case are also treated as valid.
In
DOCTYPE
must be a case-sensitive match of the string "<!DOCTYPE html>
". This is because in XHTML syntax all HTML element names are required to be in lower case, including the root element referenced inside the HTML5 DOCTYPE
.
The DOCTYPE
is optional in XHTML5 and may simply be omitted.
See also
- Document type definition contains an example
- RDFa
- XML schema
References
- ^ HTML2 HTML3 HTML4
- ^ "The HTML syntax ― HTML5". Retrieved 2011-06-05.
- ^ ISBN 0-19-853737-9.
- ^ Walsh, Norman (2001-08-06). "XML Catalogs". The Organization for the Advancement of Structured Information Standards (OASIS).
- W3C. NOTE-sgml-xml-971215.
- ^ "The DOCTYPE Declaration". Archived from the original on 2011-08-14. Retrieved 2011-09-09.
- ^ "DOCTYPE Declaration". msdn.microsoft.com.
- ^ "W3C QA - Recommended list of Doctype declarations you can use in your Web document". www.w3.org. Retrieved 22 March 2019.
- ^ "HTML Standard". html.spec.whatwg.org. Retrieved 22 March 2019.
- Web Hypertext Application Technology Working Group. Retrieved 2011-06-05.
3. A string that is an ASCII case-insensitive match for the string "DOCTYPE". 5. A string that is an ASCII case-insensitive match for the string "HTML".
- Web Hypertext Application Technology Working Group. Archived from the originalon 2012-06-18. Retrieved 2009-09-01.
- ^ "Polyglot Markup: HTML-Compatible XHTML Documents". World Wide Web Consortium. Retrieved 2012-01-17.
External links
- HTML Doctype overview
- Recommended DTDs to use in your Web document - an informative (not normative) W3C Quality Assurance publication
- DOCTYPE grid - another overview table [Last modified 27 November 2006]
- Quirks mode and transitional mode
- Box model tweaking