How to use font-weight with font-face fonts?

@font-face {
    font-family: 'DroidSerif';
    src: url('DroidSerif-Regular-webfont.ttf') format('truetype');
    font-weight: normal;
    font-style: normal;
}
@font-face {
    font-family: 'DroidSerif';
    src: url('DroidSerif-Italic-webfont.ttf') format('truetype');
    font-weight: normal;
    font-style: italic;
}
@font-face {
    font-family: 'DroidSerif';
    src: url('DroidSerif-Bold-webfont.ttf') format('truetype');
    font-weight: bold;
    font-style: normal;
}
@font-face {
    font-family: 'DroidSerif';
    src: url('DroidSerif-BoldItalic-webfont.ttf') format('truetype');
    font-weight: bold;
    font-style: italic;
}

From the tutorial: http://www.456bereastreet.com/archive/201012/font-face_tip_define_font-weight_and_font-style_to_keep_your_css_simple/