About XHTML header statement, what is DOCTYPE?

Xhtml

When we make a page, we always see a bunch of statements at its source dock, such as the most common

<! DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 transitional//en" "Http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd ">

If we get rid of the code, we'll see that the page may have a noticeable change, because it makes some styles behave differently.

The code we call the DOCTYPE declaration. DOCTYPE is a shorthand for document type, which is used to indicate what version of XHTML or HTML you are using. The DOCTYPE declaration must be placed at the top of every XHTML document, above all code and logo.

The DTD (for example, the XHTML1-TRANSITIONAL.DTD in the example above) is called the document type definition, which contains the rules of the document, and the browser interprets the identity of your page based on the DTD you define and displays it.

To create a standard Web page, the DOCTYPE declaration is an essential part of the key; unless your XHTML determines a correct DOCTYPE, your logo and CSS will not work.

XHTML 1.0 offers three types of DTD declarations to choose from:

Transition (Transitional): A very loose DTD that allows you to continue using HTML4.01 's identity (but to conform to the XHTML notation). The complete code is as follows:

<! DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 transitional//en" "Http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd ">

Strict (Strict): Requires a strict DTD, you can not use any of the performance layer of identity and attributes, such as
。 The complete code is as follows:

<! DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 strict//en" "Http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd" >

Framework (Frameset): A DTD specifically designed for frames page design, if your page contains frames, you need to use this DTD. The complete code is as follows:

<! DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 frameset//en" "Http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd" >

Ideally, of course, a strict DTD, but for most of us who have just come in contact with Web standards, the transition DTD (XHTML 1.0 Transitional) is the ideal choice now. Because the DTD also allows us to use the identity, elements, and attributes of the presentation layer, it is also easier to pass the code checksum of the consortium.

Note: The above "performance layer identification, attributes" refers to those who are purely used to control the performance of the tag tag, such as the table used for typesetting, background color identification, and some internal styles. The identity in XHTML is used to represent structs, not to implement representations, and the purpose of our transition is to ultimately achieve separation of data and performance.

Structure and performance separation. The XHTML tag in the page is simply used to represent the structure, and its representation is controlled by the CSS style. The separation of the structural performance of the standardization is also mentioned unconsciously. The header statement above is just the basics, and the basis of the page, is the first step in starting your page with XHTML.