HTML Guide for query

The pipe character | is not permitted in the query component of a URL in the href attribute of an a element.

According to the WHATWG and W3C HTML specifications, URLs in attributes such as href must be valid and properly encoded. The pipe character | is not a valid character in the query string of a URL unless it is percent-encoded as %7C. Failing to encode it will cause validation errors. This is especially important for interoperability and security across browsers and user agents.

Incorrect example (invalid href with pipe):

<a href="https://example.com/search?q=test|demo">Invalid link</a>

Correct example (pipe character encoded):

<a href="https://example.com/search?q=test%7Cdemo">Valid link</a>

Always encode special characters such as | in URLs used within HTML attributes to ensure your documents validate and behave consistently.

Space characters are not permitted in the value of the href attribute; they must be properly percent-encoded.

The href attribute specifies a URL, and URLs must follow specific syntax rules defined by RFC 3986. Spaces and some other characters are considered illegal in URLs. To include a space in the URL, use the percent escape sequence %20 in place of the space character.

Incorrect example with an illegal space in the query string:

<a href="search.html?q=my search">Search for 'my search'</a>

Correct example using percent-encoding for the space:

<a href="search.html?q=my%20search">Search for 'my search'</a>

Replace all spaces in URLs within href attributes with %20 to ensure W3C validation and proper browser behavior.

The href attribute on an element <a> contains a character that is not allowed, and should be encoded.

Some typical examples include the pipe character | that should be replaced by its encoded alternative %7C , and the left square bracket [ that needs to be encoded as %5B.