[csswg-drafts] [css-variables] Whether to support override a property using itself

helloyou2012 has just created a new issue for 
https://github.com/w3c/csswg-drafts:

== [css-variables] Whether to support override a property using itself
 ==
I want to override property like this

```
:root {
  --main-color: #333;
  --main-color: var(--main-color, #666);
}
h1 {
  color: var(--main-color);
}
```

and expect

```
h1 {
  color: #333;
}
```

Does support ?

Please view or discuss this issue at 
https://github.com/w3c/csswg-drafts/issues/876 using your GitHub 
account