In bug 1688695 we've added a pref to unship the non-standard values of
left | right | top-outside | bottom-outside
These values are not implemented by any other engine, and are not in any
specification as of today.
There was some discussion on here as of 2016 about left | right, but it
didn't gain traction:
If there is interest from other vendors to implement them, we could keep
them, but otherwise having them around complicating our table layout
code seems unfortunate.
Let me know if there's any objection or similar.
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