Bad value X for attribute “src” on element “img”: Illegal character in path segment. Space is not allowed.

Spaces in the src attribute of an img element are not valid URL characters and must be encoded as %20 or replaced with hyphens/underscores.

URLs follow the rules defined in RFC 3986, which does not allow literal space characters in any part of a URL. When a browser encounters a space in a src value, it may try to percent-encode it automatically, but this behavior is not guaranteed across all contexts. The W3C validator flags this because the HTML specification requires attribute values containing URLs to hold valid URL strings.

There are two ways to fix this: rename the file to remove spaces, or percent-encode the spaces as %20. Renaming is the better long term approach because it avoids encoding issues entirely and makes URLs easier to read and share.

Examples

Invalid: space in the file name

<img src="my photo.jpg" alt="A photo">

Valid: percent-encoded space

<img src="my%20photo.jpg" alt="A photo">

Valid: renamed file with no spaces

<img src="my-photo.jpg" alt="A photo">

Learn more:

MDN Reference

HTML img element - src attribute

→

MDN Reference

URL encoding and valid URLs

→

WHATWG Spec

HTML specification - Embedded content

→