[css-fonts] Font family selection

On Chromium, we've seen some instances where users end up with a font
family that does not contain all of the expected styles. For example,
someone might only have the bold style of Helvetica installed. When such a
user views text with a style "font-family: Helvetica, sans-serif;
font-weight: normal;", they would see bold text because that's the only
style Helvetica can display, even though the page author clearly intended
for regular text.

Currently, the font matching algorithm requires using the first font family
in the list that exists. The way the style matching is written, as long as
there is any style, that family will match. I would like to propose that
the algorithm should take the intended style (weight/stretch/etc) into
account when matching families, and allow for matching a family specified
later in the font-family rule if that family can provide a better style
match.

One possible implementation would be to assign a 'match-quality' to each
family. For each family, the match-quality is evaluated and the family with
the best match quality is used. One possible way is to evaluate
match-quality is to assign it one of three values: good, average, or poor.
A good match matches the desired style exactly or within a small tolerance
(say, using a weight 300 font instead of desired weight 400). An average
match matches the desired style on most characteristics but would be
noticeably different. A poor match would differ noticeably on multiple
criteria or have a very large difference on a single criteria (bold vs
extra-light, for example). A more fine-grained scale could also be
designed, if desired.

One thing I'm not quite sure on is italic vs oblique matching. I suspect
most users would not notice a significant difference between the two, but
it would be good to get an opinion from someone familiar with typography.

Any comments or concerns?