What is the role of the DOCTYPE statement? How is strict mode differentiated from promiscuous mode?

The HTML language has been around for too long, and there must be a few different versions of the document present, in order to make the browser clear about the version type and style of your document, you need to make the version and style of the current document at the beginning of the document with the DOCTYPE declaration. If you provide version information in a Web page, you can help verify that the code in the page conforms to the current version and style

In strict mode, the standard mode, the browser renders the page according to the, and in promiscuous mode, the page is displayed in a backward-compatible way in case the old station shop doesn't work.

If the HTML document contains a full form of doctype, it is generally rendered in standard mode, and for HTML4.01 documents containing strict DTD DOCTYPE often results in the page being rendered in standard mode. DOCTYPE not present or improperly formatted causes the document to be rendered in promiscuous mode

What is the role of the DOCTYPE statement? How is strict mode differentiated from promiscuous mode?