[css-box][css-shapes][css-masking] `half-border-box` in Box-Edge Keywords

[Crissov]

The Box module specifies several value types of box-edge keyword sets to be used in other specifications:

• <visual-box> = content-box | padding-box | border-box
• <layout-box> = <visual-box> | margin-box
• <paint-box> = <visual-box> | fill-box | stroke-box
• <coord-box> = <paint-box> | view-box

Some modules still need to extend these. In particular, css-shapes adds one type that css-masking extends to another one:

• <shape-box> = <visual-box> | margin-box | half-border-box
• <geometry-box> = <shape-box> | fill-box | stroke-box | view-box

However, both of these differ only by the addition of the peculiar half-border-box keyword from one of the predefined subsets of <box>:

• <shape-box> = <layout-box> | half-border-box
• <geometry-box> = <coord-box> | half-border-box

Therefore, I wonder whether that box-edge keyword and the value sets using it should be backported to [css-box] or if those two specialized set definitions should be simplified as shown above in their respective modules.

By the way, <display-box> from css-display is unrelated.