2.1.119 [CSS-Level2-2009] Section 17.6.3, Border styles
V0291:
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Some of the values of the 'border-style' have different meanings in tables than for other elements. In the list below they are marked with an asterisk. none No border. *hidden Same as 'none', but in the collapsing border model, also inhibits any other border (see the section on border conflicts). … *inset In the separated borders model, the border makes the entire box look as though it were embedded in the canvas. In the collapsing border model, same as 'ridge'. *outset In the separated borders model, the border makes the entire box look as though it were coming out of the canvas. In the collapsing border model, same as 'groove'.
Quirks Mode and IE7 Mode (All Versions)
The following clarifications apply:
In the collapsing border model,
hidden
behaves the same asnone
.In the collapsing border model,
inset
andoutset
behave the same as in the separated borders model.