The practical way is setting font-family to a value that is the specific name of the semibold version, such as
font-family: "Myriad pro Semibold"
if that’s the name. (Personally I use my own font listing tool, which runs on Internet Explorer only to see the fonts in my system by names as usable in CSS.)
In this approach, font-weight is not needed (and probably better not set).
Web browsers have been poor at implementing font weights by the book: they largely cannot find the specific weight version, except bold. The workaround is to include the information in the font family name, even though this is not how things are supposed to work.
Testing with Segoe UI, which often exists in different font weight versions on Windows systems, I was able to make Internet Explorer 9 select the proper version when using the logical approach (of using the font family name Segoe UI and different font-weight values), but it failed on Firefox 9 and Chrome 16 (only normal and bold work). On all of these browsers, for example, setting font-family: Segoe UI Light works OK.