Bad value X for attribute “href” on element “a”: Unexpected slash or backslash.

The URL in this href has a slash or backslash where the parser did not expect one, usually too many slashes after the scheme or a backslash standing in for the // separator.

After a scheme like https: the URL syntax expects exactly two forward slashes before the host: https://example.com. Adding a third slash (https:///example.com) leaves an empty host, and writing the separator with backslashes (https:\\example.com) uses a character the URL grammar does not allow there. In both cases the validator reports an unexpected slash or backslash.

This often comes from string concatenation that joins a base URL already ending in / with a path that also starts with /, or from copying a path that used backslashes. Browsers may paper over some of these, but the link can still resolve to the wrong place or fail in non-browser clients.

Invalid example

<a href="https:///example.com/page">Visit the page</a>

Valid example

<a href="https://example.com/page">Visit the page</a>

Keep exactly two forward slashes after the scheme, and use forward slashes throughout the URL.

Learn more:

MDN Reference

MDN - The Anchor element

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MDN Reference

MDN - What is a URL?

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WHATWG Spec

WHATWG URL Standard

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