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ValidateHTML

Common HTML Errors & How to Fix Them

Learn about the most frequent HTML mistakes developers make and how to fix them. Use our validator below to check your own code.

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The Most Common HTML Errors

1. Unclosed Tags

Every opening tag should have a corresponding closing tag. Unclosed tags can cause content to inherit unexpected styles and break your layout.

Wrong
<p>First paragraph
<p>Second paragraph
Correct
<p>First paragraph</p>
<p>Second paragraph</p>

2. Missing Alt Attributes

The alt attribute is required on all <img> elements. It provides text for screen readers, displays when images fail to load, and helps search engines understand your image content.

Wrong
<img src="hero.jpg">
Correct
<img src="hero.jpg" alt="Mountain landscape at sunset">

3. Deprecated Elements

Elements like <center>, <font>, <big>, <strike>, and <marquee> were deprecated in HTML5. Use CSS for styling instead.

Wrong
<center>Centered text</center>
<font color="red">Red text</font>
Correct
<div style="text-align: center">Centered text</div>
<span style="color: red">Red text</span>

4. Incorrect Tag Nesting

HTML has strict rules about which elements can be nested inside others. Inline elements generally cannot contain block elements.

Wrong
<a href="/"><div>Click me</div></a>
Correct
<a href="/">Click me</a>

5. Missing DOCTYPE

The DOCTYPE declaration tells the browser which version of HTML to use. Without it, browsers enter 'quirks mode' which can cause inconsistent rendering.

Wrong
<html>
<head><title>Page</title></head>
Correct
<!DOCTYPE html>
<html>
<head><title>Page</title></head>

6. Duplicate IDs

Each ID must be unique within a page. Duplicate IDs break JavaScript selectors, cause accessibility issues, and produce invalid HTML.

Wrong
<div id="main">First</div>
<div id="main">Second</div>
Correct
<div id="main">First</div>
<div id="secondary">Second</div>

Detailed Error Guides

Click any error for a detailed explanation, code examples, and step-by-step fix instructions.

Unclosed HTML Tag

Learn why unclosed HTML tags break layouts and how to fix them. Find unclosed div, p, span, and other tags with examples.

Missing Alt Attribute on Images

Learn why every img element needs an alt attribute and how to write good alt text for accessibility and SEO.

Deprecated HTML Elements

List of deprecated HTML elements like center, font, marquee and their modern CSS replacements. Fix deprecated tag warnings.

Incorrect Tag Nesting

Learn the HTML nesting rules: which elements can contain which. Fix p inside p, div inside span, and other nesting errors.

Missing DOCTYPE Declaration

Learn why <!DOCTYPE html> is required and what happens when it's missing. Fix quirks mode rendering issues.

Duplicate ID Attributes

Why duplicate IDs break JavaScript and accessibility. Learn how to find and fix duplicate id attributes in your HTML.

Missing Lang Attribute

Learn why the lang attribute on the html element is critical for screen readers, translations, and SEO.

Missing Viewport Meta Tag

Why the viewport meta tag is essential for responsive design. Fix mobile rendering issues with the correct viewport configuration.

Missing Charset Declaration

Why declaring charset UTF-8 is essential and how missing charset causes encoding issues and garbled text.

Empty Heading Tags

Why empty h1, h2, h3 headings hurt SEO and accessibility. How to fix or remove empty heading elements.

Heading Level Skip

Why skipping heading levels (h1 to h3) hurts accessibility. Learn the correct heading hierarchy for SEO and screen readers.

Excessive Inline Styles

Why inline style attributes make HTML harder to maintain. Learn when to use external CSS instead of inline styles.

Missing Page Title

Why every HTML page needs a title tag. Impact on SEO, browser tabs, bookmarks, and social sharing.

Missing Meta Description

Why meta descriptions matter for SEO click-through rates. Learn how to write effective meta descriptions for search results.

Missing Form Labels

Why every form input needs a label. How to properly associate labels with inputs for screen readers and usability.

Broken or Empty Links

How to find and fix broken links, empty href attributes, and placeholder links in your HTML code.