@property --registered {
  syntax: '*';
  initial-value: ;
  inherits: true;
}

  initial-value: ; /* Single space, right? */
  initial-value:  ; /* Maybe two spaces will work? */
  initial-value: \ ; /* Maybe escaping it will work? */
  initial-value: '\ '; /* Maybe quoting it will work? */
  initial-value: ' '; /* Oh, this is just a string "' '" */
  initial-value: " "; /* If single quotes don’t work, double ones might? */
  initial-value: 0020; /* Charcode, maybe? */
  initial-value: \0020; /* O yeah, hex must have a leaving \ */
  initial-value: "\0020"; /* Maybe quote it? */
  initial-value: \\0020; /* Escaped \ before the charcode, maybe? */
  initial-value: U+0020; /* Or a codepoint maybe? */
  initial-value: "U+0020"; /* What if I quoted it? */
  initial-value: 0000; /* This miraculously works for colors but not for anything else */
  initial-value: ​; /* A zero width space perhaps? */