@property --start-button-opacity-A {
  syntax: "<number>"; /* 単位のない数値 */
  inherits: true;
  initial-value: 1;
}

@property --start-button-opacity-B {
  syntax: "<number>";
  inherits: true;
  initial-value: 1;
}

#startButton {
  --start-button-opacity-A: 0;
  --start-button-opacity-B: 0;
  opacity: calc(var(--start-button-opacity-A) + var(--start-button-opacity-B));
}