FrontEnd Interview: What does a DOCTYPE do?
Feb 7, 2020 08:00 · 448 words · 3 minute read
Hi! Let’s answer today to one of the typical Frontend Interview questions. If this is your first time here and you want to learn Frontend and JavaScript, start right now by subscribing, and don’t forget to turn on the subtitles. [Music playing] DOCTYPE is an abbreviation for DOCument TYPE. A document type declaration is an instruction that associates a particular SGML (for example, a webpage) with a document type definition (DTD) (f.e. HTML 2.0 - 4.0) or XML document. A DTD defines how documents of a certain type should be structured (f.e.
a button can contain 00:37 - a span but not a div), whereas a DOCTYPE declares what DTD a document supposedly respects (f.e. this document respects the HTML DTD). For webpages, the DOCTYPE declaration is required. It is used to tell user agents what version of the HTML specifications your document respects. Once a user agent has recognized a correct DOCTYPE, it will trigger the no-quirks mode matching this DOCTYPE for reading the document. If a user agent doesn’t recognize a correct DOCTYPE, it will trigger the quirks mode. DOCTYPEs are required for legacy reasons.
01:14 - Including the DOCTYPE in a document ensures that the browser makes a best-effort attempt at following the relevant specifications. The DOCTYPE declaration for the HTML5 standards is <!DOCTYPE html>. HTML5 is not SGML-based. The browsers use the DOCTYPE only for mode selection. Since web browsers are implemented with special-purpose HTML parsers, rather than general-purpose DTD-based parsers, they don’t use DTDs and will never access them even if a URL is provided. The difference between HTML and XHTML is following.
01:53 - The Extensible Hypertext Markup Language, or XHTML, has two important notes for frontend developers. It needs to be well-formed, meaning all elements need to be closed and nested correctly or it returns an error. Since it is more strict than HTML is requires less pre-processing by the browser, which may improve the performance of a site. An HTML validator is a quality assurance program used to check HTML markup elements for syntax errors. A validator can be a useful tool for an HTML user who receives data electronically from a variety of input sources.
02:35 - Syntax errors, such as open tags, extra spaces, or forgotten quotation marks, can cause a Web page to look drastically different than the creator intended, or render correctly in one browser, but not in another. A valid HTML document is essential for building the Document Object Model and Accessibility Object Model. [Music playing] If you like this video give it “thumbs up”, share it with your friends, subscribe to the channel and watch other episodes. Thanks for watching and dive deeper. .