[css3-writing-modes] Proposal to unprefix writing mode keywords for width property

Now that both Webkit and Gecko have implementations of min-content,
max-content, fill-available and fit-content for width (and neither for
height), I propose we unprefix the keywords for width.

Note: Gecko unfortunately implemented an older spec as -moz-available
instead of -moz-fill-available, so unprefixing should also bring both Gecko
and Webkit into alignment.

Does anyone have any objections?

- E