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Bad value X for attribute “href” on element “a”: Percentage ("%") is not followed by two hexadecimal digits.

URLs need to be encoded so that special characters area escaped, for example space characters need to be converted to “%20”. All special characters will therefore be converted to a percent sign followed by two hexadecimal characters, to be later decoded. In case a percentage sign (%) is found without being followed by two hexadecimal digits, a decoding error will be raised.

The most probable cause is an URL that contains a % that has not been properly encoded to %25 which is the code for the percent sign. For example:

<!-- This is invalid as the percentage sign has not been properly encoded -->
<a href="https://example.com?width=48%">link</a>

<!-- This is valid as the percentage sign has been encoded as %25 -->
<a href="https://example.com?width=48%25">link</a>

Read more about the URI spec