Question on border-widths

Hi XSL editors.

I have question about "border-width" property treatment.

"border-*-width" are described as it's Intial vaule are medium.
In my understanding, it's reasonable for block-areas.
But I'm not sure about treatment for inline-areas or stacked areas. 
Why is it's inital value not zero?

If we use stacked inline-areas without specific border-width, 
should these borders be summed up simply?

e.g, ----

<block>start<inline>foo<inline>bar<inline>foo2</inline></inline></inline>end</block>

Should this fo be renderd to like below? 
(please assume "|", "+" and "-" as medium-size border)

     +---+---+----+++
     |   +---+----+++   
     |   |   +----+++   
start|foo|bar|foo2|||end
     |   |   +----+++   
     |   +---+----+++   
     +---+---+----+++

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mailto:next@tom.sfc.keio.ac.jp TOUYAMA Norio