HTML Validator
HTML Validator — runs in your browser. Inputs are not stored.
Basic HTML check
Enter values, then press Calculate.
Uses the browser DOMParser and a tag-stack heuristic. It does not replace the W3C validator.
How to use
- Paste markup to check into HTML.
- Press Check. The result area opens with a status and an issue count.
- If there are no problems, status is No issues; otherwise Issues found.
- In Details, read unclosed tags and mismatched tags line by line.
- Reset clears input and hides the result area again. There is no copy button.
Key concepts
The HTML validator is a fast heuristic that matches open and close tags on a stack—not a replacement for a standards checker. Void elements are not pushed. A close tag with nothing to match is reported as no matching open tag; a different name is a mismatch; names left at the end are unclosed tags. Parser warnings, if any, are prepended. Use it to scan pairs before you fix draft markup.
It does not check attribute validity, accessibility, or the full document-type rules. The browser’s lenient repair can differ from these results, so use an official validator before you ship. Markup is not sent to a server. If there are zero issues, the details field says the heuristic found no obvious problems. Comments and doctype declarations are skipped on the stack, so lots of comments do not inflate close errors.
Example
<div><p>HTML to check</p></div> becomes No issues and issue count 0. <div><p>text</div> without closing p stacks a mismatch or unclosed-tag line in details. The result area opens only after a check; Reset hides the detail list again.
Related: HTML formatter · HTML editor · HTML heading extractor
Frequently asked questions
Is the HTML validator the same as the W3C checker?
No. It is a fast heuristic on a tag stack and parser warnings, and does not replace the full official rules.
If status is Issues found, what do I look at?
Details gets one line per issue. Missing close pairs, name mismatches, unclosed tags, and parser warnings can be mixed.
Are void tags like img treated as errors?
If they are on the void list, they are not pushed even without a close. Only ordinary elements left unclosed become issues.
Is there a copy button?
No. Select the details text yourself. Reset hides the result area.
Is markup sent to a server?
No. The check and parser run only in the browser. There is no upload step.
What does details say when issues are 0?
A note that the heuristic found no obvious problems. Accessibility, attribute errors, and full doctype rules are better checked with an official validator.
Last reviewed: 2026-09-04