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Doctype HTML is a declaration that tells the browser what version of HTML the document is written in. This declaration appears as the very first line in an HTML file. Here's an example of an HTML file with a doctype declaration:
DOCTYPE is a declaration that tells the browser how to interpret the HTML document. It is important to use the correct DOCTYPE for different versions of HTML. Learn more about the meaning and syntax of DOCTYPE in HTML at GeeksforGeeks, a portal for computer science enthusiasts.
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The DOCTYPE for HTML5 is case-insensitive and can be written as shown below: < !DOCTYPE html > Doctype Usage: . In the version, HTML 4.01, the usage of DOCTYPE declaration was to create a reference to a document type definition (DTD), since the version HTML 4.01 was completely based on a Standard Generalized Markup Language(SGML).
The HTML document type declaration, also known as DOCTYPE, is the first line of code required in every HTML or XHTML document. The DOCTYPE declaration is an instruction to the web browser about what version of HTML the page is written in. This ensures that the web page is parsed the same way by different web browsers.
6 Answers Sorted by: 54 Traditionally, a Doctype, or Document Type Declaration associates the document with a Document Type Definition. The Document Type Definition is a standard for a specific XML or SGML document.
What Is Doctype In HTML5? Which Doctype Is Correct For HTML5
The DOCTYPE is not actually an element or HTML tag. It lets the browser know how the document should be interpreted, by indicating what version or standard of HTML (or other markup language) is being used. HTML5 Documents For HTML5 documents (which nearly all new web documents should be), the DOCTYPE declaration should be:
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The HTML !doctype tag is used for specifying which version of HTML the document is using. This is referred to as the document type declaration (DTD).. The HTML 4 document type declaration starts off with 
HTML Declaration Previous Complete HTML Reference Next Example
8 Answers Sorted by: 79 Basically, the DOCTYPE describes the HTML that will be used in your page. Browsers also use the DOCTYPE to determine how to render a page. Not including a DOCTYPE or including an incorrect one can trigger quirks mode.
The HTML 4.01 strict doctype validates against the HTML 4.01 spec, although it doesn't allow any presentational markup or deprecated elements (such as elements) or framesets to be used.
Doctype html public
The simplest and most reliable doctype declaration to use is the one defined in HTML5: If you need a doctype matching a specific version of (X)HTML, the doctype declaration must be exact (both in spelling and in case) to have the desired effect, which makes it sometimes difficult.
The HTML !DOCTYPE tag is the very first thing that every compliant web document should have. It's purpose is to inform the browser the type of document it's about to process. In HTML5, the !DOCTYPE declaration remains only for legacy reasons pertaining processing modes in browsers. The Document Type Declaration (DTD) is unique for each version.
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The goal of DOCTYPE html is to provide programmers and browsers with information at first glance of the Document Type Definition (DTD) and how a website should be rendered. The DTD indicates which code language is used on the page or in the HTML document. This applies, for example, to files in HTML, XHTML, SVG, MathML, or XML.
Simplified doctype declaration HTML5
A document type declaration, or DOCTYPE, is an instruction that associates a particular XML or SGML document (for example, a web page) with a document type definition (DTD) (for example, the formal definition of a particular version of HTML 2.0 - 4.0). In the serialized form of the document, it manifests as a short string of markup that conforms to a particular syntax.
In HTML 4.01, this declaration refers to a Document Type Definition (DTD), which specifies the structure and the legal elements of an XML document. The is declared before the tag. The declaration is not case sensitive. Syntax [Root element] [Publicity] " [Registration] // [Organization] // [Type] [Name] // [Language] " " [URL] ">
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Now, that may look like a lot of nonsense to you, so let me offer a somewhat simplified explanation of how it is constructed. For a much more detailed look at exactly what each character refers to, see the article !DOCTYPE .