Have you ever carefully understood the tag, how it is defined and how to use it_HTML/Xhtml_webpage production

Foreword:
Today I was asked, "Have you carefully understood the tag?" I was stunned for a moment, because I first saw the suggestion to use XHTML Transitional DTD on W3cschool, and then I was very confused. Obediently set Dreamweaver's HTML document type to XHTML 1.0 Transitional by default. It has been used as a matter of course ever since. It’s true that people become lazier as they grow older, and many things are subconsciously taken for granted. Since I have been asked, I went to look for information, summarized it, and took it as a note~~ The definition of

: The
statement is located in the document The frontmost position in , before the tag. This tag tells the browser which HTML or XHTML specification the document uses.
This tag can declare three DTD types, representing strict version, transitional version and framework-based HTML version respectively. (If the markup in the document does not follow the DTD specified by the doctype declaration, the document may not pass code verification and may not be displayed correctly in the browser.)

Usage of :
Parsing: In the above declaration, it is declared that the root element of the document is html, which is defined in the public identifier as "-//W3C//DTD XHTML 1.0 Strict// EN" is defined in the DTD. The browser will understand how to find a DTD that matches this public identifier. If it is not found, the browser will use the URL following the public identifier as the location to look for the DTD.
-: 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.
is the default, indicating that the organization name has been registered.
DTD: Specifies the public text class, that is, the type of object being referenced. Default is DTD.
HTML: Specify a public text description, a unique, descriptive name for the public text being referenced. A version number can be attached at the back. Default is HTML.
URL: Specifies the location of the referenced object.
Strict: Excludes all representative attributes and elements that W3C experts want to phase out.

Three HTML document types:
HTML 4.01 specifies three document types: Strict, Transitional and Frameset.
a) If you need clean markup without cluttering the presentation layer, use the HTML Strict DTD type:

b) Transitional DTD can contain presentation attributes and elements that the W3C expects to be moved into style sheets . If the user is using a browser that does not support Cascading Style Sheets (CSS) and you have to use the rendering features of HTML, use the Transitional DTD type:

c ) Frameset DTD is used with A framed document. The Frameset DTD is identical to the Transitional DTD, except that the frameset element replaces the body element:


Three XML document types:
XHTML 1.0 specifies three XML document types: Strict, Transitional, and Frameset.
a) If you need clean markup without presentation clutter, use the XHTML Strict DTD type:

b) Transitional DTD can contain presentation attributes and elements that the W3C expects to be moved into stylesheets . If the user is using a browser that does not support Cascading Style Sheets (CSS) and you have to use the rendering features of HTML, use the Transitional DTD type:

c ) Frameset DTD is used with A framed document. The Frameset DTD is equivalent to the Transitional DTD, except that the frameset element replaces the body element:


Select the correct doctype:
To get the correct doctype declaration, The key is to make the dtd correspond to the standard that the document follows.
For example, assuming that the document follows the xhtml 1.0 strict standard, the document's doctype declaration should reference the corresponding dtd.
On the other hand, if the doctype declaration specifies an xhtml dtd, but the document contains old-style html markup, it is inappropriate; similarly, if the doctype declaration specifies an html dtd, but the document contains xhtml The 1.0 strict tag is also inappropriate.
If no valid doctype declaration is specified, most browsers will use a built-in default dtd. In this case, the browser will use the built-in dtd to try to display the tag you specify (but this is what happens when the page is poorly written).
I looked at JD.com, Taobao, and Blog Park, and they all use this one (I have always used this one):

But I found that Baidu and Google use "", so I checked the information carefully and found that HTML5 also uses this directly, but because HTML 5 is not based on SGML, there is no need to modify the DTD Make
references, but require a doctype to regulate browser behavior (to make browsers behave the way they should).

It is recommended to use "" directly in the future. Using will turn on the standard compatibility mode of the browser. In the standard compatibility mode, it is not guaranteed to be compatible with other versions (before IE6, directly Ignore it), Internet Explorer remains compatible, the rendering behavior of documents may be different from future Internet Explorer, but please feel free to use it~~

Tips:
XHTML 1 is the XML version of HTML 4.01, which is a format that is not forward-compatible.
The doctype in HTML 4.01 requires a reference to the DTD because HTML 4.01 is based on SGML.
SGML specifies a standard format for embedding description tags in documents and specifies a standard method for describing the structure of a document. The HTML format currently used on the WEB is a SGML document that uses a fixed set of tags.