CSS semi-bold

HTML

<p>Normal</p>
<b>Bold</b>
<p class="semi">Semi-bold</p>
<span>Bold</span>
<span>Semibold</span>

CSS

@import url(http://fonts.googleapis.com/css?family=Open+Sans:400,600,700);
body {
    font-family: 'Open Sans';
}
.semi {
    font-weight: 600;
}