HTM_057 is reported as error by default. The EPUB breaks a rule of the specification and is invalid. Stores and Send to Kindle may reject it, and EPUBCheck exits with status 1.

EPUBCheck · XHTML content documents

`HTM_057`

Updated: Nov 3, 2026

EPUBCheck HTM_057 is an error message: “Viewport "…" value must be a positive number or the keyword "device-…".” A viewport width or height is not a positive number or the device-width/device-height keyword.

The official message

Viewport "%1$s" value must be a positive number or the keyword "device-%1$s"

Default severity: Error. The EPUB breaks a rule of the specification and is invalid. Stores and Send to Kindle may reject it, and EPUBCheck exits with status 1.
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

Values with units (px, %), zero or negative numbers, or typos.

How to fix it

Use a plain positive number, for example width=1200.

Terms used here

Other HTM messages

HTM_055 The "…" element should not be used (discouraged construct)
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_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_062 SVG "xlink:href" attribute is deprecated; an "href" attribute should be used instead.
HTM_063 SVG "xlink:href" attribute value ("…") should be equal to the "href" attribute value ("…").

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