HTML Viewer Online

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HTML Code

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How It Works

Step 1: Paste HTML code including tags, attributes, inline CSS, JavaScript, and external resource links (stylesheets, scripts, images) into the code editor.
Step 2: The viewer parses HTML structure validating syntax, identifying elements (div, span, p, img), and extracting embedded styles and scripts for rendering.
Step 3: Renders HTML in sandboxed iframe with full CSS styling, JavaScript execution (optional), and responsive layout preview for desktop/mobile viewports.
Step 4: Provides live preview with real-time updates as you type, inspect element tools, and view source toggle for debugging HTML markup and testing web components.

HTML Viewing Examples

Example: HTML Page Preview

HTML Code Input

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Product Card</title>
  <style>
    .card { border: 1px solid #ddd; padding: 20px; border-radius: 8px; max-width: 300px; }
    .card h2 { color: #333; margin: 0 0 10px; }
    .price { font-size: 24px; color: #e74c3c; font-weight: bold; }
    .btn { background: #3498db; color: white; padding: 10px 20px; border: none; border-radius: 4px; cursor: pointer; }
  </style>
</head>
<body>
  <div class="card">
    <h2>Wireless Headphones</h2>
    <p>Premium noise-cancelling headphones with 30-hour battery life.</p>
    <div class="price">$199.99</div>
    <button class="btn" onclick="alert('Added to cart!')">Add to Cart</button>
  </div>
</body>
</html>

Rendered Output Output

Rendered Preview:

╔═══════════════════════════════════╗
║  Wireless Headphones              ║
║                                   ║
║  Premium noise-cancelling         ║
║  headphones with 30-hour          ║
║  battery life.                    ║
║                                   ║
║  $199.99                          ║
║                                   ║
║  ┌──────────────┐                 ║
║  │ Add to Cart  │ ← clickable     ║
║  └──────────────┘                 ║
╚═══════════════════════════════════╝

Rendered Elements:
  <div class="card"> - Container with border, padding, rounded corners
  <h2> - Product title in dark gray (#333)
  <p> - Description text in default color
  <div class="price"> - Price in red (#e74c3c), bold, 24px
  <button class="btn"> - Blue button with white text, hover effect

CSS Applied:
  ✓ Border: 1px solid #ddd
  ✓ Padding: 20px
  ✓ Border-radius: 8px (rounded corners)
  ✓ Max-width: 300px (responsive)
  ✓ Button: Blue background, white text

JavaScript Interaction:
  ✓ Button click triggers alert: "Added to cart!"
  ✓ onclick event handler executes in sandboxed iframe

Preview Modes:
  ✓ Desktop view (default)
  ✓ Mobile view (320px width)
  ✓ Tablet view (768px width)

Key Changes:

The viewer renders complete HTML document with embedded CSS and JavaScript in real-time. The DOCTYPE declaration ensures standards mode rendering. The style block contains CSS rules applied to elements—.card class creates a bordered container with padding and rounded corners via border-radius. The h2 heading receives custom color (#333) and margin. The .price class uses large font size (24px) and red color (#e74c3c) for visual emphasis. The button (.btn) has blue background (#3498db), white text, and rounded corners, with cursor:pointer for hover feedback. The onclick attribute demonstrates JavaScript execution—clicking the button triggers an alert dialog. The viewer renders this in a sandboxed iframe, preventing malicious scripts from affecting the parent page. This is critical for safely previewing untrusted HTML from user input, email templates, or third-party widgets. Frontend developers use HTML viewers to test component markup, preview email HTML (which has limited CSS support), debug responsive layouts, and validate HTML structure before deploying to production.

Frequently Asked Questions

How does the viewer handle inline CSS, JavaScript, and external resources?

The viewer renders inline CSS from <style> tags and style attributes, executing inline JavaScript from <script> tags in a sandboxed iframe for security. External resources like <link rel="stylesheet" href="..."> and <script src="..."> are loaded if URLs are accessible (CORS-compliant). Images (<img src="...">) display if URLs are valid or use data URIs (base64-encoded images). The viewer supports modern CSS features: flexbox, grid, custom properties (CSS variables), animations, and media queries. JavaScript executes with access to DOM APIs (querySelector, addEventListener) but restricted access to parent window for security. This makes the viewer suitable for testing HTML email templates, landing pages, or widget code snippets. For external resources blocked by CORS, the viewer shows a warning suggesting to use inline styles or data URIs instead.

Can the viewer detect and highlight HTML validation errors?

Yes. The viewer validates HTML structure and highlights common errors: unclosed tags (<div> without </div>), improperly nested elements (<p><div></div></p> - block inside inline), missing required attributes (img without alt), deprecated tags (<font>, <center>), and invalid attribute values. Validation follows HTML5 specification, checking semantic correctness like <header> outside <footer>, <main> appearing multiple times, or <h1> skipping to <h3>. Error messages appear in the console panel with line numbers and suggested fixes. For example, "Line 15: Unclosed <div> tag - add </div> before </body>". The viewer also warns about accessibility issues: missing alt text on images, low color contrast, missing ARIA labels on interactive elements. This helps developers write standards-compliant HTML that passes W3C validation and improves SEO and accessibility scores.

Does the viewer support responsive design testing and mobile preview?

Yes. The viewer includes viewport controls to test responsive designs at common breakpoints: mobile (375px), tablet (768px), desktop (1024px), and custom widths. The preview iframe resizes to match selected viewport, triggering CSS media queries (@media (max-width: 768px)). You can toggle between portrait and landscape orientations. The viewer displays the current viewport dimensions and device pixel ratio. This is essential for testing responsive layouts, Bootstrap/Tailwind breakpoints, and mobile-first designs. The viewer also simulates touch events for testing mobile interactions like swipe gestures or tap targets. For advanced testing, you can enable device emulation modes (iPhone, iPad, Android) that adjust user agent strings and viewport meta tags. This helps identify layout issues before deploying to production, ensuring consistent rendering across devices.