HTM_063 is reported as usage by default. Informative only and hidden by default: it appears when EPUBCheck runs with -u (--usage). It does not make the EPUB invalid.

EPUBCheck · XHTML content documents

`HTM_063`

Updated: Nov 3, 2026

EPUBCheck HTM_063 is a usage message: “SVG "xlink:href" attribute value ("…") should be equal to the "href" attribute value ("…").” An SVG element has both xlink:href and href, with different values.

The official message

SVG "xlink:href" attribute value ("%1$s") should be equal to the "href" attribute value ("%2$s").

Default severity: Usage. Informative only and hidden by default: it appears when EPUBCheck runs with -u (--usage). It does not make the EPUB invalid.
EPUBCheck version: 5.4.0

EPUBCheck prints this ID with an underscore instead of a hyphen; this page uses the ID exactly as EPUBCheck reports it.

%1$s, %2$s… are filled in with the file, value or element EPUBCheck found.

Typical causes

One of the two attributes updated and the other forgotten.

How to fix it

Make both values the same, or keep only href.

Terms used here

Other HTM messages

HTM_062 SVG "xlink:href" attribute is deprecated; an "href" attribute should be used instead.
HTM_061 "…" is not a valid custom data attribute (it must have at least one character after the hyphen, be XML-compatible, and not contain ASCII uppercase letters).
HTM_060a EPUB reading systems must ignore secondary viewport meta elements in fixed-layout documents; viewport declaration "…" will be ignored.
HTM_060b EPUB reading systems must ignore viewport meta elements in reflowable documents; viewport declaration "…" will be ignored.
HTM_057 Viewport "…" value must be a positive number or the keyword "device-…"
HTM_055 The "…" element should not be used (discouraged construct)

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