function isEtagUsable (etag) {
  if (etag.length <= 2) {
    // Shortest an etag can be is two chars (just ""). This is where we deviate
    //  from the spec requiring a min of 3 chars however
    return false
  }

  if (etag[0] === '"' && etag[etag.length - 1] === '"') {
    // ETag: ""asd123"" or ETag: "W/"asd123"", kinda undefined behavior in the
    //  spec. Some servers will accept these while others don't.
    // ETag: "asd123"
    return !(etag[1] === '"' || etag.startsWith('"W/'))
  }

  if (etag.startsWith('W/"') && etag[etag.length - 1] === '"') {
    // ETag: W/"", also where we deviate from the spec & require a min of 3
    //  chars
    // ETag: for W/"", W/"asd123"
    return etag.length !== 4
  }

  // Anything else
  return false
}
Note: this deviates from the spec a little. Empty etags ("", W/"") are valid,
 however, including them in cached resposnes serves little to no purpose.

@see https://www.rfc-editor.org/rfc/rfc9110.html#name-etag

@param {string} etag
@returns {boolean}