Free online HTML toolkit: format, minify, validate markup, check WCAG accessibility, audit SEO, and sanitize code with client-side processing.
Loading tool...
The Online HTML Toolkit is an enterprise-grade, privacy-first developer workspace that bundles 15+ essential tools for working with HTML markup — all running directly inside your browser. Format, beautify, and indent messy code; minify production HTML; validate structural markup for errors; sanitize untrusted input; preview in sandboxed responsive viewports; audit accessibility compliance (WCAG 2.1); analyze on-page SEO factors; scan for security vulnerabilities; inspect DOM trees; extract and analyze links and images; generate modern platform components; and compare code revisions with visual diffs. Built entirely on client-side Web APIs, zero HTML code or file uploads are ever transmitted to external servers.
Executes pure in-browser parsing using standard DOMParser, custom tokenizer state machines, and isolated iframe sandboxing with strict origin boundaries. The structural validator tokenizes opening/closing element hierarchies, detecting unclosed tags, illegal nesting (such as block elements inside inline containers), duplicate IDs, missing required attributes, and deprecated elements with exact line/column pointers. The accessibility auditor implements WCAG 2.1 Level A/AA checks including heading hierarchy validation, form label association, ARIA role validation, and landmark region analysis. The SEO analyzer extracts and validates title tags, meta descriptions, canonical URLs, Open Graph metadata, Twitter cards, and JSON-LD structured data schemas. The sanitizer performs defensive AST tree-walking to dismantle script vectors, inline event handlers, and unsafe URL schemes. The live preview renders in an isolated sandbox with a custom postMessage console bridge, while the responsive engine supports device emulation across mobile, tablet, and desktop breakpoints with orientation switching.
<main><section><h1>Product Overview</h1><p>High performance tools.</p></section></main><main>
<section>
<h1>Product Overview</h1>
<p>High performance tools.</p>
</section>
</main>Accurately indents nested parent-child hierarchies while respecting void and raw-text elements. Supports 2-space, 4-space, and tab indentation.
Dialog open trigger and modal container<button type="button" commandfor="my-dialog" command="show-modal">Open Modal</button>
<dialog id="my-dialog">
<h2>Confirmation</h2>
<button type="button" commandfor="my-dialog" command="close">Dismiss</button>
</dialog>Generates declarative, zero-JavaScript native dialog modals supported across modern web browsers using the Invoker Commands API.
<div class="hero">
<h1>Welcome to Our Platform</h1>
<!-- Hero section -->
<p>Build amazing things.</p>
</div><div class="hero"><h1>Welcome to Our Platform</h1><p>Build amazing things.</p></div>Strips comments, excess whitespace, and optional closing tags. Reduces payload size to improve page load speed and Core Web Vitals scores.
Yes. The HTML Toolkit is completely free with no signup requirements, no usage limits, no watermarks, and no premium upsells. All 15+ tools are available immediately.
No. The analysis tools in the HTML Toolkit operate client-side inside your browser runtime. HTML markup and uploaded files are not uploaded to an Outlivo server. In standard preview mode, any external subresources specified within your HTML (such as external images or stylesheets) may be requested by the browser unless Strict CSP preview is active.
Yes. Live preview runs in a strictly sandboxed iframe without allow-same-origin, preventing user HTML and scripts from accessing parent session data, cookies, or any Outlivo application state.
It tokenizes elements in real time to detect tag mismatches, unclosed tags, invalid nesting rules (such as block elements inside paragraphs), duplicate IDs, and deprecated tags with precise line and column numbers.
Formatting (or beautifying) adds proper indentation, line breaks, and consistent spacing to make your HTML readable. The HTML Toolkit supports 2-space, 4-space, and tab indentation styles while preserving content inside preformatted blocks like <pre>, <code>, and <script>.
Yes. The Minify tab strips unnecessary whitespace, comments, optional closing tags, and redundant attributes to produce the smallest possible HTML payload. This improves Time to First Byte (TTFB) and Largest Contentful Paint (LCP) metrics.
The accessibility auditor checks for missing image alt text, unlabeled form controls, broken heading sequences (skipped heading levels), invalid or missing ARIA roles, missing landmark regions, keyboard-inaccessible interactive elements, and insufficient color contrast indicators.
The toolkit includes detection and code generators for native <dialog> modals, the Popover API, Declarative Invoker Commands (command / commandfor), and grouped exclusive <details name="..."> accordions — all using zero-JavaScript platform primitives.
Yes. The Diff tab allows you to paste or import two HTML revisions and see a visual line-by-line comparison showing additions (green), deletions (red), and structural DOM changes.
Yes. The SEO Audit tab extracts and validates all <script type="application/ld+json"> blocks, checks JSON syntax, identifies the Schema.org @type, and displays the full structured data payload for review.