Except that, technically speaking,
font-weight:100; is not the same as
font-weight:normal;. (Although in practical terms, it might as well be!)
Font-weight has a scale from 100 to 900 (no idea why they didn't just use 1 to 9, as it only allows multiples of 100). A weight of 400 corresponds to 'normal' and a weight of 700 corresponds to 'bold'. Other numbers on the scale can be for varying degrees of light, book, semibold, extrabold etc.
Where a font doesn't have those extra weights built in then browsers should use the nearest matching weight. I do have a handful of fonts that come in a variety of weights, but the computer doesn't recognise them as being variants of the same font, but different fonts, so the browser doesn't know that 'Gotham Book / 100' is actually 'Gotham Light', for example. I haven't yet seen an example of this working properly in practice, to give anything other than 'regular' and 'bold', except that some browsers will try to fake a semibold for
weight:500, but just like when the OS tries to fake an italic or bold face for a font that doesn't have one, these look awful.
So yes, by all means use
font-weight:100;, but there is a one-in-a-billion chance that someone, somewhere, might see it in the wrong variant of the font. If you're worried about that one-in-a-billion chance, using
font-weight:400; won't cost you an extra bytes.