HTML Validator (Syntax & Structure Checker)
Check HTML for unclosed and misnested tags, stray end tags, duplicate ids and attributes, missing alt, obsolete elements and doctype/title/charset problems.
This is a local check of syntax and structure, modelled on how browsers parse HTML. It is not the W3C Nu HTML Checker: attribute values, ARIA rules and the full list of named character references are not verified.
| Line | Column | Type | Message |
|---|---|---|---|
| 1 | 1 | Info | No <meta name="viewport"> — without it phones show the page zoomed out. |
| 9 | 40 | Error | </b> closes <b> (opened at line 9, column 18) while <i> (opened at line 9, column 31) is still open inside it — close it first. |
| 9 | 44 | Error | Stray end tag </i> — no <i> element is open. |
| 11 | 3 | Error | <img> needs an alt attribute: describe the image, or use alt="" if it is purely decorative. |
| 12 | 3 | Error | The <center> element is obsolete — use CSS (text-align or margin: auto). |
| 12 | 17 | Error | Duplicate id "info" — it is already used at line 10, column 8. Each id must be unique in the page. |
Browsers forgive broken HTML, but they repair it in ways you may not expect: a <div> inside a <p> silently ends the paragraph, misnested <b><i> tags are rebuilt, and duplicate ids break scripts and labels. This checker reads your HTML the way a browser does and lists every problem with its line and column — unclosed and stray tags, invalid nesting, duplicate attributes and ids, missing alt text, obsolete elements and attributes, and for full pages the doctype, title, language and character encoding.
How to use this tool
- Paste a full page or a snippet into the box.
- Leave Check as on Automatic, or force full-document or fragment checks.
- Read the Messages table: errors first need fixing; warnings are good practice.
- Fix the first problems and check again — one missing end tag can cause several messages.
Formula
Errors = parse errors + nesting and content-model errors + obsolete features; Warnings = recommended fixes
End tags that HTML lets you omit (</p>, </li>, </td>, </tr>, </option> and a few more) are not reported when missing. Elements inside <svg> and <math> follow XML rules, so self-closing tags are fine there. Script and style contents are not parsed as HTML.
Example
The sample page has five problems. <b>Monday to <i>Friday</b></i> gives two errors: </b> closes the bold text while the italic text is still open, and </i> then has nothing left to close. The <img> has no alt text, <center> is obsolete, and the id info is used twice. That is 5 errors and no warnings.
Frequently asked questions
Is this the same as the W3C validator?
No. It runs locally in your browser and covers the most common syntax and structure problems. The W3C Nu HTML Checker also verifies every attribute value, ARIA usage and much more; use it for a final, complete check.
Why is a missing </p> not an error?
HTML allows the end tag of p, li, td, tr, option and some other elements to be left out; the browser closes them automatically in well-defined places.
Why does <div> inside <p> cause an error at the </p>?
A paragraph cannot contain block elements, so the browser ends the <p> when it sees <div>. The </p> later on then has no paragraph to close.
Is my HTML uploaded?
No, the check runs entirely in your browser.
Sources
Last updated:
Page address with these inputs: https://kakao.mobile/tools/html-validator/
Cite or embed this tool
kakaoCalc. (2026, November 8). HTML Validator (Syntax & Structure Checker). https://kakao.mobile/tools/html-validator/
“HTML Validator (Syntax & Structure Checker).” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/html-validator/.
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