[css-images-4][css-color-5] color-mix() and cross-fade() have different serialization order?

```
color-mix() = color-mix( <color-interpolation-method> , [ <color> && <percentage [0,100]>? ]#{2})
```
```
cross-fade() = cross-fade( <cf-image># )
<cf-image> = <percentage [0,100]>? && [ <image> | <color> ]
```

The `color-mix()` and `cross-fade()` functions have different grammar orders wrt progress and value, they should probably be synchronized.