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ValidateHTML

Incorrect Tag Nesting

HTML has strict rules about which elements can be nested inside others. Block elements (div, section, article) cannot be placed inside inline elements (span, a, strong). Paragraphs (<p>) auto-close when they encounter another block element, causing unexpected behavior.

Why It Matters

Incorrect nesting causes browsers to silently restructure your DOM, often producing a completely different tree than what you wrote. This leads to CSS rules not matching, JavaScript selectors failing, and layout bugs that are extremely hard to debug.

Code Examples

❌ Invalid
<p>
  <div>This breaks the paragraph</div>
</p>

<span>
  <p>Block inside inline</p>
</span>
✓ Valid
<div>
  <p>Paragraph text</p>
  <div>Block content</div>
</div>

<div>
  <p>Block inside block</p>
</div>

How to Fix

  • 1<p> cannot contain block elements like <div>, <section>, <ul>, or another <p>.
  • 2<span> cannot contain block elements — use <div> as the parent instead.
  • 3<a> can contain block elements in HTML5, but avoid deeply nested structures.
  • 4Use your browser DevTools to inspect the actual DOM — it shows how the browser restructured your HTML.

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