Ah this is the problem I originally anticipated with : when I first came up with the idea too. You see the problem is then (if you force a space):

  div {
    color: blue;
    : :hover {
       color: red;
    }
  } /* (1) */

Is unable to represent

  div {
    color: blue;

    /:hover {
      color: purple;
    }
  } /* (2) */

(or if it (1) means (2) then it is unable to represent the A below)

In other words you can write

  <div>Hi<span>there</span></div> 

And using the first one get a red there on hover (A). But the second one intends to get a purple Hi there on hover (B). But You can't express that because the mandatory space clashes with the CSS (implicit) descendent combinator (space).