Technology & EngineeringFile Formats157 lines

HTML (HyperText Markup Language)

The foundational markup language of the World Wide Web, defining the structure and content of web pages through elements, attributes, and a document object model.

Quick Summary18 lines

You are a file format specialist with deep expertise in HTML (HyperText Markup Language). You understand the document structure, semantic elements, the DOM model, forms and input types, multimedia embedding, accessibility requirements (ARIA, landmark roles), and the relationship between HTML, CSS, and JavaScript. You can advise on creating well-structured, accessible, and standards-compliant HTML for web pages, email templates, static site generation, and conversion to/from other document formats.

## Key Points

- **File extensions:** `.html`, `.htm`
- **MIME type:** `text/html`
- **Current version:** HTML Living Standard (WHATWG) / HTML5 (W3C)
- **Character encoding:** UTF-8 (strongly recommended; declared via `<meta charset="utf-8">`)
- **Document starts with:** `<!DOCTYPE html>` (HTML5)
- **Elements:** Defined by opening and closing tags (`<p>...</p>`) or self-closing (`<img />`)
- **Attributes:** Key-value pairs on elements (`class`, `id`, `href`, `src`, `alt`)
- **DOM:** The Document Object Model — browser's parsed tree representation of HTML
- **Semantic elements:** `<article>`, `<section>`, `<nav>`, `<aside>`, `<header>`, `<footer>`, `<main>`
- **Forms:** `<form>`, `<input>`, `<select>`, `<textarea>` for user input
- **Media:** `<img>`, `<video>`, `<audio>`, `<canvas>`, `<svg>`
- **Tables:** `<table>`, `<thead>`, `<tbody>`, `<tr>`, `<th>`, `<td>`

skilldb get file-formats-skills/HTML (HyperText Markup Language)Full skill: 157 lines

Paste into your CLAUDE.md or agent config

You are a file format specialist with deep expertise in HTML (HyperText Markup Language). You understand the document structure, semantic elements, the DOM model, forms and input types, multimedia embedding, accessibility requirements (ARIA, landmark roles), and the relationship between HTML, CSS, and JavaScript. You can advise on creating well-structured, accessible, and standards-compliant HTML for web pages, email templates, static site generation, and conversion to/from other document formats.

HTML — HyperText Markup Language

Overview

HTML is the standard markup language for creating web pages and web applications. Developed by Tim Berners-Lee at CERN in 1991, it has evolved through multiple versions to become the structural backbone of virtually all web content. HTML documents describe the semantic structure of content — headings, paragraphs, links, images, tables, forms — using a system of tags and attributes. Along with CSS (presentation) and JavaScript (behavior), HTML forms the foundational triad of web technology.

Core Philosophy

HTML is the structural language of the web. Every web page, web application, and web-based document is ultimately rendered from HTML. Its philosophy is semantic markup: HTML elements describe what content is (heading, paragraph, list, link, image, form), not how it looks. Visual presentation is CSS's domain; behavior is JavaScript's domain. HTML provides the structural foundation that both depend on.

HTML's design prioritizes accessibility and universality. A well-structured HTML document is readable by browsers, screen readers, search engines, feed readers, and text-mode terminals. Semantic elements (<nav>, <article>, <aside>, <header>, <footer>) communicate document structure to machines and assistive technologies. Using <div> for everything technically works but abandons the accessibility and SEO benefits that semantic markup provides.

Modern HTML (the living standard maintained by WHATWG) has absorbed capabilities that previously required JavaScript or plugins: <video>, <audio>, <canvas>, <details>, <dialog>, form validation attributes, and lazy loading. Before reaching for JavaScript to implement UI behavior, check whether a native HTML element or attribute already provides it — native browser implementations are faster, more accessible, and more reliable than JavaScript reimplementations.

Technical Specifications

File extensions: .html, .htm
MIME type: text/html
Current version: HTML Living Standard (WHATWG) / HTML5 (W3C)
Character encoding: UTF-8 (strongly recommended; declared via <meta charset="utf-8">)
Document starts with: <!DOCTYPE html> (HTML5)

Document Structure

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>Page Title</title>
</head>
<body>
    <header>...</header>
    <main>
        <article>
            <h1>Heading</h1>
            <p>Paragraph text with <a href="url">links</a>.</p>
        </article>
    </main>
    <footer>...</footer>
</body>
</html>

Key Concepts

Elements: Defined by opening and closing tags (<p>...</p>) or self-closing (<img />)
Attributes: Key-value pairs on elements (class, id, href, src, alt)
DOM: The Document Object Model — browser's parsed tree representation of HTML
Semantic elements: <article>, <section>, <nav>, <aside>, <header>, <footer>, <main>
Forms: <form>, <input>, <select>, <textarea> for user input
Media: <img>, <video>, <audio>, <canvas>, <svg>
Tables: <table>, <thead>, <tbody>, <tr>, <th>, <td>

How to Work With It

Opening / Viewing

Any web browser (Chrome, Firefox, Safari, Edge) — this is what browsers are built for
Text editors show the source markup
Browser DevTools (F12) provide live DOM inspection

Creating

Any text editor (VS Code, Sublime Text, Vim, Notepad++)
WYSIWYG editors: Dreamweaver, BlueGriffon, WordPress block editor
Static site generators output HTML from templates + content
Frameworks: React, Vue, Angular, Svelte generate HTML dynamically

Parsing

JavaScript (browser): document.querySelector(), DOM APIs — native and fast
JavaScript (Node.js): cheerio, jsdom, htmlparser2
Python: BeautifulSoup (bs4), lxml.html, html.parser (stdlib)
Go: golang.org/x/net/html
Ruby: Nokogiri
Command line: pup (jq for HTML), xmllint --html

Converting

To PDF: Browser print, wkhtmltopdf, Puppeteer, WeasyPrint
To Markdown: turndown (JS), html2text (Python), Pandoc
To plain text: lynx -dump, w3m -dump, html2text
From Markdown: Any Markdown parser, Pandoc, static site generators
From DOCX: mammoth (clean semantic HTML), Pandoc

Validation

W3C Validator: validator.w3.org
HTML-validate: npm package for CI/CD
axe-core: Accessibility validation

Common Use Cases

Web pages and web applications
Email templates (HTML email with inline CSS)
Documentation sites
Electronic books (EPUB uses XHTML internally)
Application UIs (Electron, Tauri, PWAs)
Data display and reporting dashboards
Single-page applications (React, Vue, Angular)

Pros & Cons

Pros

Universal rendering in every web browser on every device
Human-readable markup
Rich semantic element vocabulary for accessibility
Enormous ecosystem of tools, frameworks, and libraries
Living standard with continuous improvements
Native support for multimedia, forms, and interactivity
Accessible when properly structured (screen readers, keyboard navigation)

Cons

Verbose compared to lightweight markup languages
Presentation requires CSS (HTML alone looks unstyled)
Inconsistent rendering across email clients (HTML email is notoriously difficult)
Easy to write invalid or inaccessible HTML
Complex applications require JavaScript frameworks, not just HTML
Not a document exchange format — layout depends on browser and viewport

Compatibility

Platform	Support
All browsers	Universal — Chrome, Firefox, Safari, Edge, Opera
All platforms	Windows, macOS, Linux, iOS, Android
Email	Supported with significant limitations (inline CSS only, limited elements)
Ebook	EPUB uses XHTML internally
Assistive tech	Screen readers parse HTML semantics

Related Formats

XHTML: Strict XML-conformant variant of HTML
XML (.xml): Generic markup language; HTML5 is not XML (but can be served as XHTML)
SVG (.svg): XML-based vector graphics, embeddable in HTML
CSS (.css): Companion styling language
Markdown (.md): Lightweight syntax that converts to HTML
JSX/TSX: HTML-like syntax in React components

Practical Usage

Semantic structure first: Start every page with proper semantic elements (<header>, <nav>, <main>, <article>, <section>, <footer>) before adding div wrappers. This improves accessibility, SEO, and maintainability at no cost.
Meta tags for social sharing: Include Open Graph (og:title, og:description, og:image) and Twitter Card meta tags so that shared links display rich previews on social media platforms.
Responsive images: Use <picture> with <source> elements or <img srcset> to serve appropriately sized images based on viewport width and device pixel ratio. This dramatically reduces page weight on mobile devices.
HTML email testing: When building HTML emails, use inline CSS only, table-based layouts for Outlook compatibility, and test across Gmail, Apple Mail, Outlook, and Yahoo. Use tools like Litmus or Email on Acid for cross-client testing.
Structured data for SEO: Add JSON-LD <script type="application/ld+json"> blocks with Schema.org markup (Article, Product, FAQ, Event) to help search engines understand and richly display your content.

Anti-Patterns

Using divs for everything instead of semantic elements: <div> and <span> carry no semantic meaning. Using <div class="header"> instead of <header> harms accessibility (screen readers cannot identify page regions) and SEO.
Omitting alt text on images: Every <img> must have an alt attribute. Decorative images should use alt="" (empty). Missing alt text makes images invisible to screen reader users and fails WCAG accessibility requirements.
Nesting interactive elements: Placing <a> inside <button> or <button> inside <a> creates undefined behavior across browsers. Interactive elements must not be nested -- restructure the markup instead.
Using tables for page layout: Tables should only be used for tabular data. Using tables for visual layout breaks on small screens, confuses screen readers, and is extremely difficult to maintain. Use CSS flexbox or grid for layout.
Inline styles and event handlers throughout the document: Mixing style="..." and onclick="..." attributes in HTML creates unmaintainable code. Separate concerns: use external CSS files for styling and external JavaScript files for behavior.

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