Definition
The process of checking HTML markup against the W3C specification to catch syntax errors, unclosed tags, and structural problems that cause inconsistent cross-browser rendering and harm SEO/accessibility.
Why It Matters
It ensures that code follows international standards, preventing cross-browser bugs and making websites more resilient to future technology changes. Validation is the “quality control” of web development, ensuring that your work is robust, accessible, and professional.
Core Concepts
<!-- Invalid HTML example -->
<p>This is a paragraph
<div>This is a div inside a paragraph (Illegal)</div>
</p>
<!-- Validated HTML5 -->
<!DOCTYPE html>
<html lang="en">
<head><title>Title</title></head>
<body><p>Validated content.</p></body>
</html>- Validator Tool: html5.validator.nu checks documents against HTML5 rules.
- Common Errors: Missing closing tags, missing angle brackets, incorrect file paths.
- Tag Leakage: Unclosed tags cause formatting to bleed into subsequent content.
- Benefits: More consistent rendering, better accessibility tree, improved search indexing.