#546 · Developer Tool

Canonical Tag Checker

Inspect canonical link markup in an HTML document and compare it with the page URL you expect to publish. The checker finds rel=canonical tokens regardless of attribute order, reports missing or duplicate tags, resolves relative href values, and checks the final URL’s protocol, fragment, credentials, and self-reference. It also notes common normalization differences such as host case, default ports, and trailing slashes without pretending to crawl the target.

Developer Input

Web metadata input
Ad space

How to use this developer tool

  1. Paste the requested source text or load a local file.
  2. Set any comparison values or processing options shown below the input.
  3. Select the primary action or press Ctrl/Cmd + Enter.
  4. Review the diagnostics, then copy or download the output.

What this developer tool does

The report shows the resolved canonical URL and any markup or URL comparison issues.

HTML is parsed with DOMParser. The first canonical href is resolved against the supplied page URL, then normalized with the URL API for comparison.

A matching tag does not prove that search engines will select it; redirects, internal links, sitemap entries, and content signals also matter.

Example

Input

<!doctype html>
<html><head>
<link rel="canonical" href="https://example.com/docs/page/">
<title>Example</title>
</head><body></body></html>

Run the sample to see the exact report and metrics produced by the current implementation.

Use cases

  • Pre-deployment review of generated site files
  • Debugging crawler or link-preview configuration
  • Auditing metadata during a migration
  • Creating repeatable QA evidence for a release

Tips for reliable output

  • Paste the complete source when grouping or document context matters.
  • Use absolute public URLs unless the field explicitly accepts a path.
  • Retest after redirects, route rules, or locale mappings change.
  • Keep a deliberately invalid sample for regression checks.
  • Confirm the deployed result with the relevant external platform.

Processing details

HTML is parsed with DOMParser. The first canonical href is resolved against the supplied page URL, then normalized with the URL API for comparison. The parser runs entirely in the current browser tab and returns copy-ready text plus structured exports when useful.

A matching tag does not prove that search engines will select it; redirects, internal links, sitemap entries, and content signals also matter.

Frequently asked questions

Can I use this tool without uploading data?

Yes. Processing happens in your browser, and the page makes no request with the supplied input.

What happens when the input is malformed?

The Canonical Tag Checker stops, displays an actionable error, and does not produce a misleading successful result.

Does the result guarantee search engine behavior?

No. The result checks the supplied markup or rules; crawlers and social platforms may apply additional policies and cached data.

Can I download the result?

Yes. Use Download for the primary output, or the CSV and JSON buttons when structured exports are available.

How should I test the result before deployment?

Run a representative valid sample and a deliberately invalid case, then verify the deployed public URL with the relevant crawler or platform tool.

Checks and output

AreaBehavior
InputProcessed locally
ErrorsReported without silent correction
ExportCopy, download, and structured data when available

Web Meta & Server Files

Inspect crawler directives, sitemap files, canonical URLs, localized alternates, and social metadata.

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