Syntax Checker & Code Validator
About HTML Syntax Checker
The HTML5 Syntax & Markup Validator checks HTML code for unclosed tags, illegal attribute nesting, missing `alt` attributes, unescaped entities, and broken DOM nesting structures (e.g. `<div>` inside `<p>`).
How to Use HTML Syntax Checker
Step 1
Paste your HTML markup or template snippet.
Step 2
Review the live markup health status and validation warnings.
Step 3
Click on any flagged line to inspect and correct the HTML tag.
Step 4
Click "Copy Report".
Practical Use Cases for HTML Syntax Checker
Validating HTML Templates for CMS & Email Newsletters
Catch unclosed `<div>` and `<table>` tags that break responsive web page layouts and distort HTML email newsletters.
Accessibility & SEO Markup Audit
Locate missing image `alt` attributes, duplicate `id` attributes, and invalid HTML5 heading hierarchies (`<h1>` to `<h6>`).
Input & Output Examples
Detecting Unclosed `<div>` Tag
`<div class="wrapper"><p>Hello World</p>`
Status: ⚠️ Warning | Message: `Unclosed tag '<div>' detected; missing closing '</div>' before end of document.`
Key Features & Performance
- ✓Validates HTML5 semantic tags (`<header>`, `<main>`, `<article>`, `<section>`, `<dialog>`).
- ✓Detects duplicate HTML element IDs across the document.
- ✓Checks for proper self-closing void elements (`<img />`, `<br />`, `<input />`).
- ✓100% Client-Side DOMParser & HTML tree inspection.
- ✓1-Click Copy HTML validation report.
Key Terminology & Definitions
Void Elements (HTML5)
Elements that cannot have child nodes and do not require closing tags (e.g. `<img>`, `<br>`, `<input>`, `<meta>`, `<link>`).
DOM Hierarchy Violation
Nesting block-level elements inside inline or phrasing-content containers forbidden by the HTML specification (e.g. `<div>` inside `<span>`).