Weight

The numbers are a CSS and OpenType convention rather than a measurement, and they are not comparable between families: one foundry’s 300 is another’s 400, and a face with only two weights may put them at 400 and 700 or at 300 and 600. The only way to know what a weight looks like is to look at it.

What matters in use is the distance between the weights you pick, not the weights themselves. A heading in 600 above body text in 400 barely reads as a contrast at small sizes; 700 against 400 does. And using a weight the font does not have is the one thing to avoid — the browser will fake it by smearing the outlines, which is visibly worse than simply using the weight that exists.