Respuesta :
The first line of any HTML document must include a "Document Type Declaration," also known as "doctype," which informs the browser of the page's HTML version.
The document can be rendered in standards mode by the browser thanks to this declaration.
Browsers display the page in quirks mode without it.
If you haven't heard of quirks mode, it was introduced by browsers to make pages written in an "old style" compatible with new features and standards. Without it, older pages would lose their appearance as browsers and HTML changed, and the Web Platform has always been very protective in this regard.
Unless the page is written for standards mode, browsers basically default to quirks mode. Because the DTD declaration is nearly impossible to remember, those doctype declarations always required you to save it somewhere. Additionally, there were distinct DTDs for transitional mode and strict mode, which was less stringent.
HTML4 and lower is an SGML application, whereas XHTML is an XML vocabulary. Although HTML5 is not an SGML application and has abandoned many of the strict rules of XHTML, it is heavily influenced by HTML4.
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