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HyperText Markup Language — the standard markup for web pages. Supports both text diff and structural DOM tree diff.
Tim Berners-Lee created HTML (HyperText Markup Language) at CERN in 1991 as the publishing language of the World Wide Web, and it has evolved through decades of standardization into the foundational technology that defines the structure of every web page. The WHATWG now maintains the HTML Living Standard, continuously updated rather than versioned, with major features including semantic elements (article, nav, section, aside), native form validation, audio and video embedding, the Canvas API for 2D graphics, Web Components for custom elements, and extensive accessibility attributes (ARIA). HTML documents form a tree structure — the DOM (Document Object Model) — where elements nest within elements, attributes modify behavior, and text nodes contain content.
This tree structure makes HTML uniquely suited to structural comparison that understands element hierarchy, attribute changes, and content modifications independently. HTML serves as the output format for server-side rendering frameworks (Next.js, Nuxt, Rails, Django), the source format for static site generators (Hugo, Astro, Eleventy), and the delivery format for email templates. The accessibility implications of HTML structure are significant — heading hierarchy, landmark elements, alt text, and form labels directly affect screen reader navigation and legal accessibility compliance.
Modern HTML integrates with CSS for presentation and JavaScript for interactivity, but the markup itself carries semantic meaning that search engines, assistive technologies, and content aggregators depend on. HTML validation through the W3C validator and linting tools like HTMLHint helps maintain standards compliance across large web applications.
HTML changes affect accessibility, SEO, rendering, and user interaction simultaneously, making structural comparison essential for catching the full impact of modifications. Comparing HTML files reveals broken element nesting that invalidates the DOM tree, removed alt attributes that create accessibility violations, changed form structures that break submissions, and modified heading hierarchies that degrade SEO.
Structural HTML diff matches elements by tag and position, separating attribute changes from content changes and nesting modifications — providing clarity that text-based diff cannot achieve for deeply nested markup.
UtraDiff performs both text-level source diff and structural DOM tree comparison on HTML files. The DOM diff matches elements by tag name, id, and class attributes, isolating moved sections from content edits. Attribute changes, text node modifications, and structural rearrangements are highlighted individually in the tree view.
The text diff with HTML syntax highlighting runs in parallel. Cross-format comparison supports diffing HTML against XML and SVG within the markup compatibility group.
Supported extensions: .html .htm