HTTP headers are key/value pairs sent at the beginning of a request or response. According to the grammar in RFC 7230, a field could have an empty value. In practice, it probably doesn’t make much sense: semantically, a header with an empty value or the absence of that header are equivalent. However, some client or server implementations actually require that a given header is present, even if it’s empty. For instance, the validation tests for WOPI (an HTTP-based protocol used to integrate Office for the Web with an application) require that the X-WOPI-Lock header is included in the response in certain situations, even if it’s empty (even though the spec says it can be omitted).