1. Definition
The DOCTYPE tag is a standard universal markup language document type declaration, the purpose is to tell the standard universal A markup language parser, what document type definition (DTD) it should use to parse the document.
The declaration must be the first line of the HTML document, before the tag.
2. Function
Declare the parsing type of the document (document.compatMode) to avoid the browser’s weird mode.
document.compatMode:
BackCompat: Weird mode, the browser uses its own weird mode to parse and render the page.
CSS1Compat: Standard mode, the browser uses the W3C standard to parse and render the page.
This attribute will be recognized and used by the browser, but if your page does not have a DOCTYPE declaration, then compatMode defaults to BackCompat. The browser parses and renders the page in its own way. Then, in different The browser will display different styles.
If you add to your page, then it is equivalent to turning on the standards mode, then the browser must honestly parse and render the page in accordance with W3C standards, in this way, your page will appear the same in all browsers.
This is what does.
3. Usage
HTML 5:
HTML 4.01 Strict
This DTD contains all HTML elements and attributes, but does not include presentational and deprecated elements (such as font). Framesets are not allowed.
HTML 4.01 Transitional
This DTD contains all HTML elements and attributes, including presentational and deprecated elements (such as font). Framesets are not allowed.
HTML 4.01 Frameset
This DTD is equivalent to HTML 4.01 Transitional, but allows frameset content.
XHTML 1.0 Strict
This DTD contains all HTML elements and attributes, but does not include presentational and deprecated elements (such as font). Framesets are not allowed. Markup must be written in well-formed XML.
XHTML 1.0 Transitional
This DTD contains all HTML elements and attributes, including presentational and deprecated elements (such as font). Framesets are not allowed. Markup must be written in well-formed XML.
XHTML 1.0 Frameset
This DTD is equivalent to XHTML 1.0 Transitional, but allows frameset content.
XHTML 1.1
This DTD is equivalent to XHTML 1.0 Strict, but allows for additional models (such as providing ruby support for East Asian languages).
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