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How Our HTML Viewer Works

Overview

Our HTML Online Viewer provides a secure environment for previewing HTML code by combining advanced security measures with real-time rendering technology. The system processes your HTML code entirely in your browser, ensuring privacy while protecting against malicious code execution.

Security Architecture

1. Content Security Policy (CSP)

We implement strict Content Security Policy headers that define allowlists of trusted content sources. This prevents the browser from loading any resources from non-approved sources, effectively blocking many types of XSS attacks.

Content-Security-Policy: default-src 'self'; script-src 'none'; object-src 'none';

This policy blocks all external scripts and objects

2. HTML Sanitization

Before rendering, all HTML code goes through a comprehensive sanitization process that removes potentially dangerous elements and attributes:

  • Script Tags: All <script> tags are completely removed
  • Event Handlers: Attributes like onclick, onload, etc. are stripped
  • JavaScript URLs: javascript: and vbscript: protocols are blocked
  • Data URLs: data: protocol is restricted to prevent data injection

3. Sandboxed Iframe Execution

The sanitized HTML is rendered inside a sandboxed iframe with restricted permissions:

<iframe sandbox="allow-same-origin" />

This sandbox prevents script execution while allowing basic HTML rendering

Processing Pipeline

Step-by-Step Process:

  1. Input Reception: User enters or uploads HTML code
  2. Syntax Validation: Basic HTML structure validation
  3. Security Scanning: Identify potentially dangerous elements
  4. Content Sanitization: Remove or neutralize security threats
  5. Iframe Preparation: Create secure rendering environment
  6. Safe Rendering: Display sanitized HTML in sandboxed iframe
  7. Real-time Updates: Process changes with debounced updates

Security Measures in Detail

XSS Prevention

Cross-Site Scripting (XSS) is one of the most common web vulnerabilities. Our system prevents XSS through:

  • Script Removal: All script tags are stripped before rendering
  • Event Handler Blocking: HTML event attributes are removed
  • URL Protocol Filtering: Dangerous protocols like javascript: are blocked
  • Content Isolation: Rendered content cannot access parent page context

Code Injection Protection

We protect against various forms of code injection:

Blocked Elements

  • • <script> tags
  • • <object> and <embed> tags
  • • Event handler attributes
  • • JavaScript URLs
  • • Data URLs with scripts

Allowed Elements

  • • Standard HTML tags
  • • CSS styling
  • • Images and media
  • • Forms (non-functional)
  • • Text and formatting

Technical Implementation

Client-Side Processing

All HTML processing happens entirely in your browser using JavaScript:

// Sanitization example
const sanitizeHTML = (html) => {
  return html
    .replace(/<script[^>]*>.*?</script>/gi, '')
    .replace(/on\w+\s*=\s*["'][^"']*["']/gi, '')
    .replace(/javascript:/gi, '')
}

Real-Time Updates

The preview updates automatically as you type, with intelligent debouncing to prevent excessive processing:

  • Debounced Updates: 500ms delay to prevent excessive re-rendering
  • Incremental Processing: Only changed content is re-processed
  • Error Handling: Graceful handling of malformed HTML

Privacy & Data Handling

Your HTML code never leaves your browser. All processing, sanitization, and rendering happens locally:

Privacy Guarantees

  • • No server-side processing of your HTML code
  • • No data transmission to external servers
  • • No storage of your code on our systems
  • • Complete client-side operation

Limitations

While our security measures are comprehensive, they do impose some limitations:

  • No JavaScript Execution: Scripts are removed for security
  • Limited External Resources: External scripts and some resources are blocked
  • Form Functionality: Forms cannot submit data
  • Interactive Elements: Some interactive features may not work

Best Practices

To get the most out of our HTML viewer:

  • Focus on HTML structure and CSS styling
  • Use inline CSS for styling instead of external stylesheets
  • Test responsive designs using browser developer tools
  • Use our tool for layout and visual testing, not functionality testing

Our HTML viewer strikes the perfect balance between functionality and security, allowing you to preview HTML safely while maintaining the visual fidelity of your designs.