line-height: normal and multiple descendant font sizes

In CSS2.1, we have the following in Section 10.8.1:

"When an element contains text that is rendered in more than one font, user
agents may determine the 'normal'
'line-height'<http://www.w3.org/TR/CSS2/visudet.html#propdef-line-height>
value
according to the largest font size."

I'm curious if any UA actually implements this. I have tested Chrome,
Opera, and Safari UAs, and none of these UAs seems to use the largest font
size (of descendant fonts).

My test consisted of the following fragment:

<p style="line-height: normal">
<span style="font-size: 12pt; border: 1px solid #C0C0C0">X</span></br>
<span style="font-size: 36pt; border: 1px solid #C0C0C0">X</span></br>
<span style="font-size: 18pt; border: 1px solid #C0C0C0">X</span></br>
</p>

If a UA implements the above language, then I would expect that each of the
three lines be assigned a single line height (of 36pt), where the leading
on each line is computed according to the following language:

"Still for each glyph, determine the leading L to add, where L =
'line-height' <http://www.w3.org/TR/CSS2/visudet.html#propdef-line-height>
 - AD. Half the leading is added above A and the other half below D, giving
the glyph and its leading a total height above the baseline of A' = A + L/2
and a total depth of D' = D + L/2."

Also, I should note that "the largest font size" is ambiguous in the above
language, since it might mean:

   1. largest font size of descendant non-replaced elements, i.e.,
   descendants only;
   2. largest font size of descendant non-replaced elements and element (on
   which line-height of 'normal' is specified), i.e., descendants and self.

G.