[css3-grid-layout] <track-breadth> shouldn't allow negative values

Hi everyone,

in the current specification:

<track-breadth> => <length> | <percentage> | <fraction> | min-content
| max-content

As <length> and <percentage> can be any <number>, this means that the
<track-breadth> and by extension grid-row-definition and
grid-column-definition can be negative.

Talking this trough, we couldn't find a useful use case for allowing
negative grid-*-definition and thus would recommend disallowing this
by adding the following to the specification:

Negative values for <track-breadth> are illegal.

Any thoughts on this?

Thanks,
Julien