line-height calculation

i assume this is an old issue but, nevertheless, i want
to ask for it (again).

why was it chosen to calculate the line-height property
on the _largest_ font size in a line?

Quote from css2:
"When an element contains text that is rendered in
more than one font, user agents should determine
the 'line-height' value according to the largest font size."

This causes (or can cause) text with inline-formulae to
change the line height. E.g. when you try to use the
collapsed summation symbol (i found i in Donald Knuth's
"The Art of Computer Programming" which is written in
(La)Tex).

Maybe one of you can explain me. I have to admit that
I couldn't test to write inline-formulae since i found no
browser supporting mathML but it didn't work with different
font sizes. e.g: A { line-height:1.4; font-size:10pt; }
A B { line-height:1.167; font-size:12pt; }
maybe it's due to my UA (MSIE) but if i understand the
css standard right it isn't

regards
Thomas