Logical properties support

[jhpratt]

Plugin link, spec link (WD)

This is currently a stage 3 proposal in cssdb. There are plenty of examples of uses in the links above, so I won't repeat them here.

If wanted, I can try to figure out how to add a plugin myself and submit a PR.

[jonathantneal]

Possibly resolved by #445

[MoOx]

postcss-cssnext has been deprecated in favor of postcss-preset-env. Read more at https://moox.io/blog/deprecating-cssnext/