<div class='test'></div>

@property --round {
  
  initial-value: 100;
  syntax: '<integer>';
  inherits: false;
}

.test {
   background-color: #456;
   height: 100px;
   --round: 400.5;
   /* make unitless into pixels */
   width: calc((var(--round) * 1px) / 1);
}