Doctype_html/css_WEB-ITnose

Doctype in HTML has two main purposes.

1. Verify the validity of the document

It tells the user agent and validator what DTD this document is written according to. This action is passive. Every time the page is loaded, the browser does not download the DTD and check the validity. It is only enabled when the page is manually verified.

2. Determine the browser's rendering mode

For actual operations, inform the browser which parsing algorithm to use when reading the document. If it is not written, the browser will parse the code according to its own rules, which may seriously affect the html layout. Browsers have three ways to parse HTML documents.

Standard Mode

Weird Mode

Almost Standard Mode

Related Links:

Mode? Standard!

Enable browser mode through Doctype

CS002: DOCTYPE and browser mode analysis

KB001: Compatibility issues and browser kernel and rendering mode

In-depth understanding of browser compatibility mode