Specifying Style and Weight for Google Fonts
Here's the issue: You can't specify font weights that don't exist in the font set from Google. Click on the SEE SPECIMEN link below the font, then scroll down to the STYLES section. There you'll see each of the "styles" available for that particular font. Sadly Google doesn't list the CSS font weights for each style. Here's how the names map to CSS font weight numbers:
Thin 100
Extra Light 200
Light 300
Regular 400
Medium 500
Semi-Bold 600
Bold 700
Extra-Bold 800
Black 900
Note that very few fonts come in all 9 weights.
They use regular CSS.
Just use your regular font family like this:
font-family: 'Open Sans', sans-serif;
Now you decide what "weight" the font should have by adding
for semi-bold
font-weight:600;
for bold (700)
font-weight:bold;
for extra bold (800)
font-weight:800;
Like this its fallback proof, so if the google font should "fail" your backup font Arial/Helvetica(Sans-serif) use the same weight as the google font.
Pretty smart :-)
Note that the different font weights have to be specifically imported via the link tag url (family query param of the google font url) in the header.
For example the following link will include both weights 400 and 700:
<link href='fonts.googleapis.com/css?family=Comfortaa:400,700'; rel='stylesheet' type='text/css'>
For CSS2
<link href='fonts.googleapis.com/css2?family=Comfortaa:wght@400;700'; rel='stylesheet' type='text/css'>