HTML

Last updated April 23, 2026

HTML (HyperText Markup Language) is the markup language that describes the structure of a web page. It uses nested tagged elements — <h1>, <p>, <a>, <div> — each with optional attributes, to express headings, paragraphs, hyperlinks, images, forms, and everything else that makes up a browser-rendered document.

One-line history

HTML was introduced by Tim Berners-Lee in 1990 alongside the first web browser and web server. It went through versions 1.0 through 4.01 at the W3C, then a brief diversion as XHTML (HTML rewritten as strict XML), then was taken over by WHATWG as HTML5 and its successor “HTML Living Standard”. The current spec is maintained at html.spec.whatwg.org.

Relationship to XML

HTML borrows the tag-based syntax from SGML, the ancestor of both HTML and XML. It is not a valid XML dialect by default — HTML permits unclosed <img> tags, unquoted attribute values, and a handful of other deviations that a strict XML parser rejects. XHTML was an attempt to align them; in practice, browsers tolerate both and “XML-style HTML” looks slightly archaic today.

A minimal document

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Hello</title>
  </head>
  <body>
    <h1>Hello, world</h1>
    <p>A minimal HTML page.</p>
  </body>
</html>

The pieces:

<!DOCTYPE html> — the HTML5 document type declaration (see DOCTYPE).
<html> — the root element.
<head> — metadata, title, linked stylesheets, scripts.
<body> — the visible content.

Why people reach for an HTML beautifier

Pretty-printing HTML reveals structural bugs the same way pretty-printing JSON does. A missing </div> is much more obvious in a beautifier’s indented output than in a minified blob. HTML beautifiers must be aware of void elements (<br>, <img>, <hr>) that never have closing tags, inline elements that should not be broken onto new lines, and the special parsing rules for <pre> and <script>.

Common traps

Void elements have no closing tag. <br /> (XHTML style) is valid HTML but unnecessary; <br> alone is correct.
Attribute values without special characters do not need quotes, but it is a good habit to quote them all.
Character entities — &, <, >, " are the only ones you need to know for mechanical escaping. There are hundreds more for specific glyphs.
HTML is not XML — beautifiers that assume strict XML rules will corrupt perfectly valid HTML.

A JSON/XML/HTML beautifier like the one on this site treats each format on its own terms.

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