Does it give the same results as the official W3C validator?

No. This tool is a practical checker focused on common issues — mismatched tags, duplicate ids, missing alt, etc. — and isn't a full HTML5 spec validation.

What does it flag as a warning?

It warns about a missing DOCTYPE, deprecated tags (center, font, etc.), a missing lang attribute, and missing alt on img elements.

Is the code I enter sent to a server?

No. It's checked entirely in your browser.

Does "no errors" mean perfect HTML?

It means none of the frequent issues were found — not full spec compliance. Use the W3C validator for a complete check.

Should warnings be fixed too?

alt and lang directly affect accessibility and SEO, and deprecated tags are better replaced with CSS.

🔍 HTML5 Validator

Checks for common HTML issues like mismatched tags, duplicate ids, and missing alt attributes.

⚠️ Practical checker — not a full spec validation at the level of the official W3C validator

HTML Code

How to use

Enter your HTML code.
Click "Check".
Issues like mismatched tags, duplicate ids, and missing alt are listed.

🎯 Use Cases

Catch mismatched tags after finishing markup
Find duplicate id attributes
Check whether image alt attributes are missing

Example

Input

<!DOCTYPE html>
<html>
<img src="logo.png">
<p id="main">A</p>
<p id="main">B</p>
</html>

Output

❌ 5번째 줄: id="main" 가 4번째 줄과 중복됩니다
⚠️ 2번째 줄: <html> 에 lang 속성이 없습니다
⚠️ 3번째 줄: <img> 에 alt 속성이 없습니다

The duplicate id="main" is reported as an error (❌), while the missing lang on <html> and the missing alt on <img> are reported as warnings (⚠️), each with the line number where the issue occurs. Messages are shown in Korean.

What it checks and at what level

A practical checker focused on frequent mistakes. Errors (❌) should be fixed immediately; warnings (⚠️) affect accessibility and compatibility.

Check	Level	Meaning
Mismatched tags	Error	Unclosed or mis-nested tags — a common cause of broken layouts
Duplicate id	Error	Same id used twice — breaks scripts and anchors
Missing img alt	Warning	Alternative text needed for screen readers and SEO
Missing DOCTYPE	Warning	May trigger quirks-mode rendering
Deprecated tags	Warning	Obsolete tags like center and font
Missing html lang	Warning	Unspecified document language — bad for translation and search

FAQ

Does it give the same results as the official W3C validator?: No. This tool is a practical checker focused on common issues — mismatched tags, duplicate ids, missing alt, etc. — and isn't a full HTML5 spec validation.
What does it flag as a warning?: It warns about a missing DOCTYPE, deprecated tags (center, font, etc.), a missing lang attribute, and missing alt on img elements.
Is the code I enter sent to a server?: No. It's checked entirely in your browser.
Does "no errors" mean perfect HTML?: It means none of the frequent issues were found — not full spec compliance. Use the W3C validator for a complete check.
Should warnings be fixed too?: alt and lang directly affect accessibility and SEO, and deprecated tags are better replaced with CSS.