Re: [csswg-drafts] [css-grid] fr unit and other length units

The CSS Grid Layout spec actually doesn't define the `fr` unit as a 
_length unit_ of any kind. Instead, it introduces the new dimension 
type — "Flexible Length", which can currently be used in one context 
only — for a [size of a grid 
track](https://drafts.csswg.org/css-grid/#typedef-track-breadth), 
which is defined as
> `<track-breadth>       = <length-percentage> | <flex> | min-content 
| max-content | auto`

(where `<flex>` is this new dimension type).

Since [the `width` 
property](https://drafts.csswg.org/css-box-3/#propdef-width) is 
defined as
> `[<length> | <percentage>] | available | min-content | max-content |
 fit-content | auto`

which doesn't allow `<flex>` type of value, `width: 1fr` would be an 
invalid declaration, just like `width: 1deg`, `width: 500Hz` and so 
on. I don't see any ambiguity in the current definition

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