[css-tables] Firefox table caption left and right for caption-side

[gregwhitworth]

Gecko supports caption-side properties of the left and right. Our desire is to add this to the next level as currently no other vendor implements this, thus we shouldn't try and gain interop on this, but it would be useful to authors.

[FremyCompany]

Resolved we should leave the note about the left/right and *-outside values, but not define them normatively