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EPUBCheck · XHTML content documents

EPUBCheck HTM-025: unregistered URI scheme

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EPUBCheck HTM-025 is a warning message: “Non-registered URI scheme type found in href.” A link uses a URI scheme that is not registered. EPUBCheck warns because reading apps may not know what to do with it; the book stays valid.

The official message

Non-registered URI scheme type found in href.

Default severity
Warning. The EPUB is still valid, but something is likely to cause problems in some reading apps. EPUBCheck exits with 0 unless you pass --failonwarnings.
EPUBCheck version
5.4.0

Typical causes

  • A typo in the scheme, such as htp: or http;.
  • An app-specific scheme copied from a website or a document management system.
  • A Windows path or a local file reference written as a link.

How to fix it

  1. Open the file EPUBCheck names (the message gives the path, line and column) in a text or EPUB editor such as Sigil, or unzip the EPUB and open it there.
  2. Correct the scheme to http, https, mailto or another standard one.
  3. If the link only worked inside one app or on the author's computer, remove the link and keep the text.
  4. Rebuild the EPUB and run EPUBCheck again to confirm the message is gone.

When it's not this

If the link is empty (href=""), that is HTM-045, not HTM-025.

Terms used here

Other HTM messages

  • HTM-011 Entity is undeclared.
  • HTM-010 Namespace "…" is unusual for the prefix "epub".
  • HTM-009 The DOCTYPE provided is obsolete or irregular and can be removed.
  • HTM-007 Empty or whitespace-only value of attribute ssml:ph.
  • HTM-044 Namespace uri "…" was included but not used.
  • HTM-045 Encountered empty href.

Sources

Message text and severity from EPUBCheck 5.4.0 (W3C, BSD-3-Clause). Explanation by EpubCC.

FAQ

What does EPUBCheck HTM-025 mean?

EPUBCheck HTM-025 is a warning message: “Non-registered URI scheme type found in href.” A link uses a URI scheme that is not registered. EPUBCheck warns because reading apps may not know what to do with it; the book stays valid.

Is HTM-025 an error?

HTM-025 is reported as warning by default. The EPUB is still valid, but something is likely to cause problems in some reading apps. EPUBCheck exits with 0 unless you pass --failonwarnings.