DOCTYPE element detailed explanation complete version

1. Overview

This article systematically explains the DOCTYPE element. At the same time, I checked a lot of information. Because the information on the Internet is messy, I redefined it after collecting and sorting it out. For example, the definition of the DOCTYPE element is mainly divided into basic knowledge and advanced knowledge. Basic knowledge explains basic DOCTYPE knowledge. A lot of advanced knowledge comes from the Internet, mainly some practical application skills.

II. Definition

DOCTYPE is the abbreviation of Document Type. The <!DOCTYPE> element is used to declare the document type definition (DTD) of a page. This element declaration is located at the very beginning of the document, before the <html> tag. By confirming the DTD of a page, you can also determine which W3C specification the page uses (such as HTML or XHTML specification).

3. W3C Specifications

The correct translation of W3C specification should be W3C Recommendations. In the eyes of many designers, W3C is the standard. But many people only know a little about it. The following is the HTML specification and XHTML specification in the current W3C specification. The relationship between HTML and XHTML will be explained later:

W3C HTML specifications and timeline

specification	recommend
HTML 3.2	January 14, 1997
HTML 4.0	May 24, 1998
HTML 4.01	December 24, 1999

W3C XHTML Specification and Timeline

specification	Draft/Proposal	recommend
XHTML 1.0		January 26, 2000
XHTML 1.0 Revision		August 1, 2002
XHTML 1.1		May 31, 2001
XHTML Modules		April 10, 2001
XHTML Modules 1.1	July 5, 2006
XHTML Basic		December 19, 2000
XHTML Basic 1.1	July 5, 2006
XHTML Events		October 14, 2003
XHTML Events 2	February 16, 2007
XHTML Print		September 20, 2006
XHTML Media Types	August 1, 2002
XForms 1.0		October 14, 2003
XForms 1.0 (SE)		March 14, 2006
XForms 1.1	February 22, 2007
XHTML 2.0	July 26, 2006
XLink		June 27, 2001
HLink	September 13, 2002

XHTML can be considered the latest HTML specification and is an XML application that can be smoothly migrated from HTML 4.01. The W3C's first step in refactoring HTML 4.01 into XML resulted in XHTML 1.0. XHTML 1.0 relies on the semantics provided by HTML 4.01 tags.

4. Specifications and DTD

The page file declares different DTDs through the <DOCTYPE> element to tell the browser which HTML or XHTML specification the current page complies with. The following only lists the DTDs related to HTML4.01 and XHTML1.0:

HTML

HTML 4.01 specifies three document types: Strict, Transitional, and Frameset.

HTML Strict DTD

Use this type if you need clean markup, free from the clutter of the presentation layer. Please use with Cascading Style Sheets (CSS):

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "
http://www.w3.org/TR/html4/strict.dtd">

HTML Transitional DTD

Transitional DTDs can contain presentation attributes and elements that the W3C expects to be moved into style sheets. Use this type if your readers use browsers that do not support Cascading Style Sheets (CSS) and you are forced to use the rendering features of HTML:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "
http://www.w3.org/TR/html4/loose.dtd">

Frameset DTD

The Frameset DTD should be used for documents with frames. The Frameset DTD is identical to the Transitional DTD, except that the frameset element replaces the body element:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "
http://www.w3.org/TR/html4/frameset.dtd">

XHTML

XHTML 1.0 specifies three XML document types: Strict, Transitional, and Frameset.

XHTML Strict DTD

Use this type if you need clean markup, free from the clutter of the presentation layer. Please use with Cascading Style Sheets (CSS):

<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

XHTML Transitional DTD

Transitional DTDs can contain presentation attributes and elements that the W3C expects to be moved into style sheets. Use this style if your readers use browsers that do not support Cascading Style Sheets (CSS) and you are forced to use the rendering features of XHTML:

<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

XHTML Frameset DTD

Use this DTD when you want to use frames!

<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">

5. DOCTYPE Element Syntax

grammar

HTML Top-Level Elements Availability "Registration // Organization // Type Tag // Definition Language" "URL"

Syntax element description

Top-level element: Specifies the top-level element type declared in the DTD. This corresponds to the declared SGML document type. The default is HTML.

Availability: Specifies whether the Formal Public Identifier (FPI) is a publicly accessible object or system resource. The value can be PUBLIC or SYSTEM.PUBLIC by default. Represents a publicly accessible object. SYSTEM represents system resources, such as local files or URLs.

Registered: Specifies whether the organization is registered with the International Organization for Standardization (ISO).

+ is the default, indicating that the organization name has been registered.

- indicates that the organization name is not registered. The Internet Engineering Task Force (IETF) and the World Wide Web Consortium (W3C) are not registered ISO organizations.

Organization: Specifies the name of the group or organization responsible for the creation and maintenance of the DTD referenced by the !DOCTYPE declaration, ie, the OwnerID. IETF is IETF. W3C is W3C.

Type: Specifies the public text class, that is, the type of object being referenced. The default is DTD.

Label: Specifies the public text description, which is a unique, descriptive name for the referenced public text. It can be followed by a version number. The default is HTML.

Definition: Specifies the document type definition.

Frameset Frameset document.

Strict excludes all the typical attributes and elements that W3C experts want to phase out because style sheets are already well established.

Transitional contains all content except the frameSet element.

Language: Specifies the public text language, that is, the natural language encoding system used to create the referenced object. The language definitions are written as ISO 639 language codes (uppercase two letters). EN Default. English.

URL: Specifies the location of the referenced object.

6. Inspection tools

If you want to check whether your page content meets the standards declared in the DOCTYPE, you can use the validation tool provided by W3C:

http://validator.w3.org/

7. DOCTYPE switch

Modern browsers include different rendering modes, with the goal of supporting both web pages that follow W3C standards and web pages designed for older browsers. Among them, Standards mode (also known as strict rendering mode) is used to render web pages that comply with the latest standards, while Quirks mode (also known as loose rendering mode or compatibility mode) is used to render web pages designed for traditional browsers. Also, note that Mozilla/Netscape 6 has a new Almost Standards mode that supports pages designed for an older version of the standards.

In theory, this should be a pretty intuitive switch. If the <!DOCTYPE> element of the page indicates the standard that the page complies with (such as XHTML1.0), the browser will switch to Standards mode. If no doctype is specified, or if HTML 3.2 or older is specified, the browser switches to Quirks mode. This allows browsers to correctly display documents that follow standards without completely discarding old, non-standard web pages. But there are the following situations:

1. Missing URL or relative URL

A complete doctype declaration includes the URL of the corresponding document type definition (DTD) file. If the URL is missing, or is specified as a relative path (rather than a fully qualified Internet address), most browsers will enter Quirks mode, regardless of the mode specified by the doctype declaration.

2. Wrong doctype

Browsers are very sensitive to the form and format of doctype declarations, and if they cannot recognize an incorrectly formed doctype, they will be forced into Quirks mode (it is recommended to copy and paste a known correct doctype into the document rather than typing it in yourself). A common cause of a malformed doctype is a missing space between the first part of the doctype and the URL. Collapsing a two-line doctype onto a single line often loses that space.

3. Transitional doctype

Inconsistencies are most likely to occur when browsers handle transitional doctypes. IE and Opera use Standards mode; Netscape 6 and older versions of Safari use Quirks mode; Netscape 7, Mozilla 1, and newer versions of Safari use Netscape's Almost Standards mode, which is a more fault-tolerant version of Standards mode.

4. Unknown doctype

Browsers are also inconsistent in how they handle doctypes they don't recognize. IE and Opera will go into Standards mode; in other words, they assume that the unrecognized doctype is a new standard that has not yet been integrated into the browser. Netscape 6, on the other hand, switches to Quirks mode when it encounters an unrecognized doctype.

Switching doctypes can be an effective way to get your browser into the correct rendering mode and display your web pages correctly, provided you are aware of the inconsistencies between browsers and proactively avoid problems.

8. XHTML usage tips

1. Immediately following the DOCTYPE declaration above is an XHTML namespace declaration, placed in the enhanced <html> element, written as:

2. Since XHTML 1.0 pages are legal XML documents, and XML is case-sensitive for tags and attributes, for simplicity, all tags and attributes in XHTML 1.0 pages must be lowercase.

Some free tools, such as HTML Tidy (http://tidy.sourceforge.net/), can help you automatically convert tags and attributes to lowercase.

3. Declare the language used in the page by adding a <meta> element to the <head> element.

4. In XHTML, all attributes must be enclosed in quotes.

Some free tools, such as HTML Tidy (http://tidy.sourceforge.net/), can help you automatically add quotes to all attributes.

5. In XHTML, all attributes must have values.

You can't write it like in HTML 4.0:

Instead, write:

6. In XHTML, all tags must be closed.

There are two ways to close a tag. A tag containing content is closed with an end tag, and an empty tag is closed with a space and "/" at the end. For example:

<p>This is acceptable HTML and it is also valid XHTML.</p>

7. Do not use "--" in comments.

"--" can only be used at the beginning and end of an XHTML comment, and cannot appear within the body of a comment. The following are not allowed:

8. HTML encode all special symbols.

W3C's XHTML/CSS/DOM specifications form a complete and rigorous system. I call these three specifications the "incarnation of the Trinity" in the Web world. These three specifications represent the structure, presentation, and behavior of a web page. Strictly dividing web pages into these three layers and making the content of each layer as independent as possible will help improve the reusability and modularity of the pages and significantly reduce the cost of page production, maintenance, and modification. In order to achieve the above-mentioned hierarchical goals, the XHTML you write should only contain tags (elements and attributes) related to the structure. Therefore, you should get used to using Strict type DTD, and abandon those tags with presentational meanings as soon as possible (these tags are marked as Deprecated in the HTML 4.0 specification, and will be completely abandoned in later versions of XHTML); abandon the old method of table-based layout as soon as possible, and adopt complete CSS layout.

9. Recommended books on XHTML

The Definitive Guide to HTML and XHTML, by Chuck Musciano & Bill Kennedy.

XHTML Tutorial, by Chelsea Valentine & Chris Minnick.

"Refactoring the Website" by Jeffrey Zeldman.
Author: Zhang Ziqiu Source: http://www.cnblogs.com/zhangziqiu/

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