Hi,
are there plans to add padding-inside and padding-outside to accompany margin-inside and margin-outside? Or has it already been done but does not show up in the documentation?

Not done yet, but it's a good idea. I'll add it to the roadmap.

likewise with "border-inside" and "border-outside", pretty please?

+1 for -outside and -inside properties for padding and border. I see it's still on the roadmap – any idea whether it's likely soon?

Meanwhile, in case it's useful to others, I've written up my hacky workaround for borders: https://gist.github.com/arthurattwell/60f46206a95b4dc2e39dcc08b83b2c4f

Essentially, I position a coloured pseudo-element behind the element I want a border-inside for, and then shift that pseudo-element inside or outside slightly using margin-inside or margin-outside. It feels inelegant and fragile, but it's working for me for now.

The relative properties are an issue we look at from time to time, most recently in the context of vertical writing where left and right are not always clearly defined (do they refer to the directions on the page, or the directions relative to the rotated text). So hopefully we will be able to figure out a way forward on this.

Did you come up with a solution?