'-vnd' identifiers

Hi,
I heard that future versions of CSS might support vendor-specific extensions
by allowing identifiers to begin with '-'. That would allow vendor-specific
properties such as -wap-input-format (WAP Forum extension) or
-xyz-inner-border-color (xyz company extension).
I wonder how that could be syntactical possible, since it would then be
impossible for a tokenizer to separate negative numbers from identifiers.
For example, is '-123' a negative number or is it an identifier?

Thanks, 
- Mats