If I open a template that contains property group and include there my font-weight with unusual number (for e.x. 300) it automatically converts to XenForo default, i.e. Font-weight: normal. Sometimes I get font-weight: bold. I think the same thing will happen with font-weight: light.
well I use the "word" parameters vs the digits..
they seem to work correctly.
normal | bold | bolder | lighter | inherit
this always worked for me...
i never touched the digits because I think I read somewhere that most fonts only have few weights available .
so if the weight doesnt exist, it should default to the next available, but sometimes depending on browser it could give unpredictable results.
and if you want to use 300, it will still display 400, if it is the only one available. (this is font dependant)
but you are right, adding "px" to the value is definetively wrong.