[CSS21] 'Minimal' line-height

I've been bashing my head against the spec for a while on this one, and now
to finally bite the bullet and ask a question.

The section in question is 10.8 "Line height calculations: the 'line-height'
and 'vertical-align' properties"

More specifically, this sentence from the line-height property explanation
in subsection 1:
"On a block-level, table-cell, table-caption or inline-block element whose
content is composed of inline-level elements, 'line-height' specifies the
minimal height of line boxes within the element."

My question is this: So this 'minimal height of line boxes'... what will
cause a height of a line box to increase?

Example 1:
<div style="line-height: 10px; font-size 12px">&nbsp;</div>
This particular font's 'glyph' for the non-breaking space is 15px in height.
Should the line-box (and, thus, the div's box) be 15px? 12px? or 10px?

And for the love of all that is bitwise-NORed, why?

Example 2:
<div style="line-height: 10px; font-size 12px"><img
width="40px" height="15px;" src="<someplace>"></div>
Same questions. Especially the "Why?"

All help and contributions welcomed and appreciated.

Style,

Chris H-C