[css-fonts-5] Consider making font-weight: bolder (and lighter) font-specific

[litherum]

Consider a font family with a weight=300 member and a weight = 600 member. Consider the following content:

<div id="outer" style="font: 300 16px MyFontFamily;">
    <div id="inner" style="font-weight: bolder;">...</div>
</div>

The definition of bolder says that the computed weight for inner would be 400. According to the font matching rules, if the requested weight is 400, a candidate font of weight 300 would be chosen rather than another candidate of weight 600.

This means that, for the inner element, the author has requested a bolder weight, but they didn't get one, even though a bolder weight exists.

Maybe bolder should take into account which members of the family are present, so this situation would match the weight=600 font rather than the weight=300 font.

[dbaron]

It's worth noting that bolder and lighter used to be defined this way, but we decided to change to the current definition. I don't recall all of the reasoning (although there was certainly a bunch of complexity around the computed values, since they had to represent "3 weights lighter than 800", etc.), but it's probably worth going through the discussions leading to that switch.

[litherum]

@jfkthame @drott

[svgeesus]

IIRC in the past we argued that bolder and lighter are blunt instruments and that if content authors really care what they get, they should specify (or calculate) explicit weights.