In CSS, for the font-weight property, the value: normal defaults to the numeric value 400, and bold to 700.
If you want to specify other weights, you need to give the number value. That number value needs to be supported for the font family that you are using.
For example you would define semi-bold like this:
font-weight: 600;
Here an JSFiddle using ‘Open Sans’ font family, loaded with the above weights.