[css2] Find out if line-heigh:normal and not normal use the same A and D

[frivoal]

Follow up form the following two comments from @dbaron in issue #1802:
#1802 (comment)
#1802 (comment)

We need to look into whether browsers use the same A and D metrics when line-height is normal vs when it is not, and whether there's a difference between the first available font and other fonts.

[fantasai]

@faceless2 provided some interesting information related to this in #2228 (comment) btw