Detailed introduction to declaration in html
The
declaration is not an HTML tag; it is used to tell the web browser which version of HTML is used for the page.
The declaration is located at the very beginning of the document, before the tag. The declaration tells the browser's parser what document standard to use to parse this document (standard model). If declared incorrectly or not declared, the document will be rendered in compatibility mode/mixed mode.
tag has no closing tag. declarations are not case-sensitive. Always add the declaration to your HTML document first to ensure the browser knows the document type in advance.
In html4.01, the statement is requiredQuoteDTD (Document Type Declaration), because it is based on SGML (Standard Generalized Markup Language Standard Generalized Markup Language), DTD specifies the rules of the markup language to ensure that the browser can correctly The rendering content. html5 is not based on SGML and therefore does not need to reference a DTD.
HTML 5 has only one declaration:<!DOCTYPE html>HTML 4.01 specifies three different declarations, namely: Strict, Transitional and FramesetHTML 4.01 StrictDoes not allow the use of expressive, deprecated elements (such as font) and
frameworksets (such as frameset).
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">HTML 4.01 TransitionalAllows the use of expressive, obsolete elements (such as font), but does not allow the use of framesets (such as frameset).
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">HTML 4.01 FramesetAllows presentational elements, exhaust elements, and framesets.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
HTML ELEMENTS AND VALID DOCTYPES
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