HTML5 Validator
Check HTML for syntax errors or formatting errors
Features
Validate against the HTML5 standard
Check syntax and conventions with the recommended html-validate rules
Classify errors and warnings
Tally counts by severity and list the problem spots
Pinpoint locations by line and column
Show the line number, column number, and an excerpt of the relevant tag for each issue
Group similar errors together
Group identical issues and aggregate them with counts
How to use
Enter the HTML code
Type or paste the HTML source code into the input field; you can also load a sample
Press the check button
Press the check button and the server parses the HTML and shows the results
Review the flagged spots
Check the line numbers and details from the list of errors and warnings, and apply fixes
Use cases
Quality check before publishing a page
Find unclosed tags and invalid nesting before publishing and correct the markup
Learning and understanding HTML
Learn correct writing rules from the issues and improve your coding skills
Verifying coding deliverables
Validate the syntactic correctness of received HTML to objectively assure quality
Maintaining existing pages
Periodically check revised markup for broken structure and attribute mistakes
Basic Knowledge
Why Validate HTML
Browsers display pages by inferring missing markup even when there are minor syntax errors, so errors often remain invisible in the rendered output. Ignoring them can lead to layout breaks across different environments and misinterpretation by search engines and screen readers.
Difference Between Errors and Warnings
Errors indicate violations of HTML specifications, while warnings point to non-conforming but discouraged syntax. The standard approach is to first eliminate all errors, then review warnings individually and decide whether to fix each one.
Safety and Privacy
Server-side processing
Input is processed on the server; only information necessary for the inquiry is sent.
Temporary processing only
For HTML validation, temporary processing occurs on the server, then data is discarded after results are returned.
Communications encrypted with HTTPS
All sent and received data is encrypted by TLS and cannot be read by third parties.
No transmission to third parties
Input content is not sent to external services such as ad networks or trackers.
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Updates
- update #380
Change error display colors in HTML5 validators and similar tools for better readability
- update #203
HTML5 validator now includes explanations of problems and how to fix them with each violation
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Can be used for commercial purposes such as research, calculation, processing, and conversion in business and production.
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Link to RAKKOTOOLS in the description, or Mention or display that you are using RAKKOTOOLS in the stream or video.
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Usage guidelinesYou are free to use it for normal purposes. However, use for purposes that violate laws, infringe on third-party rights, impose significant load on the service, or otherwise interfere with service operation is prohibited.
We do not guarantee the accuracy or completeness of information and calculation results obtained from each tool.
This notice is a translation of the original Japanese text. If there is any discrepancy between the translation and the original, the Japanese original takes precedence.