What the hell is this?

What the heck is this?

I talked about with my friends two days ago. Today I will sort out the information I learned online here

Because of the popularity of HTML5, people have slowly weakened their understanding of , but now many interviewers are still asking this question, so it is still necessary to understand it in order to have something to talk about in the future.

1. Does it have any magical effect?

To clarify this problem, let’s first explain the browser’s “standard mode” and “weird mode”.

　What is "weird mode"? To put it simply, it is a page rendering mode produced by the browser in order to be compatible with web pages that were designed for older versions of browsers and did not strictly follow W3C standards. At this time, the rendering engine will also switch to the required version.

　What is "standard mode"? When the user needs to display some new pages that meet W3C specifications, the rendering engine switches to a "standard mode" opposite to the "weird mode". In this mode, the rendering engine is the latest version. This also meets more W3C specifications.

　Finally, “standard mode” and “weird mode” are collectively called the browser’s document mode.

　In the era of IE6, in order to comply with W3C standards and ensure that old web pages can continue to be browsed, "standard mode" and "weird mode" were created. Both rendering modes exist in the same browser, but manual switching is impractical. Microsoft proposed the document type (DOCTYPE), a very "novel" concept at the time

　 In this way, "gorgeous" was born. Let's not talk about its advantages and disadvantages. Let's just talk about its function: telling the browser's parser what document standard to use to parse this document.

2. Will it be affected if does not exist or is in the wrong format?

　 DOCTYPE that does not exist or is incorrectly formatted will cause the document to be rendered in "weird mode".

3.Why does HTML5 only need to write

　HTML5 is not based on SGML, so there is no need to reference DTD (Document Type Definition), but doctype is needed to regulate browser behavior (let browsers run the way they should).

　And HTML4.01 is based on SGML, so it needs to reference the DTD (Document Type Definition) to inform the browser of the document type used in the document.

Note: SGML is an international standard for defining electronic documents and content descriptions. The basic idea is to separate the content and style of the document. It consists of SGML Declaration, Document Type Definition (DTD) and Document Instance.

Reference source:

　http://www.cnblogs.com/zichi/p/5147159.html