All browsers support the declaration.
Definition and Usage
The declaration must be the first line of the HTML document, located in < before the ;html> tag. (Recommended study: HTML Getting Started Tutorial)
declaration is not an HTML tag; it is an instruction that instructs the web browser about which HTML version to use for writing the page.
In HTML 4.01, the declaration references a DTD because HTML 4.01 is based on SGML. A DTD specifies the rules for a markup language so that browsers can render content correctly.
HTML5 is not based on SGML, so there is no need to reference a DTD.
Tip: Always add the declaration to your HTML document so the browser knows the document type.
Differences between HTML 4.01 and HTML5
There are three types of declarations in HTML 4.01. There is only one in HTML5:
<!DOCTYPE html>
Instance
<!DOCTYPE html>文档的标题 文档的内容......
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