Re: [fxtf-drafts] [motion] default reference box for basic shape

I think that defining the `border-box` as the default reference box for `<basic-shape>` value seems reasonable for `offset-path`.

If there is the border of the containing block where the offset path will be drawn, it’ll be expected to position the box along it. Also, the `border-box` reference box helps keep the offset path synchronized with the border if margins are used.
The `margin-box` reference box would be useful when it's important to wrap the containing block around with the offset path and influence the area beyond the dimensions of the containing block. But `offset-path` doesn’t need to change the relationship of other boxes around the containing block.


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