Google Web Fonts determines the browser with User Agent Sniffing. Here are some examples.
For Google Chrome it returns a WOFF font:
$ curl -A 'Mozilla/5.0 (Windows NT 6.2; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/32.0.1667.0 Safari/537.36' http://fonts.googleapis.com/css?family=Open+Sans:400,700
@font-face {
font-family: 'Open Sans';
font-style: normal;
font-weight: 400;
src: local('Open Sans'), local('OpenSans'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/cJZKeOuBrn4kERxqtaUH3T8E0i7KZn-EPnyo3HZu7kw.woff) format('woff');
}
@font-face {
font-family: 'Open Sans';
font-style: normal;
font-weight: 700;
src: local('Open Sans Bold'), local('OpenSans-Bold'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/k3k702ZOKiLJc3WVjuplzHhCUOGz7vYGh680lGh-uXM.woff) format('woff');
}
For Internet Explorer 10 it returns an EOT and a WOFF font:
$ curl -A 'Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)' http://fonts.googleapis.com/css?family=Open+Sans:400,700
@font-face {
font-family: 'Open Sans';
font-style: normal;
font-weight: 400;
src: url(http://themes.googleusercontent.com/static/fonts/opensans/v6/cJZKeOuBrn4kERxqtaUH3fY6323mHUZFJMgTvxaG2iE.eot);
src: local('Open Sans'), local('OpenSans'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/cJZKeOuBrn4kERxqtaUH3fY6323mHUZFJMgTvxaG2iE.eot) format('embedded-opentype'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/cJZKeOuBrn4kERxqtaUH3T8E0i7KZn-EPnyo3HZu7kw.woff) format('woff');
}
@font-face {
font-family: 'Open Sans';
font-style: normal;
font-weight: 700;
src: url(http://themes.googleusercontent.com/static/fonts/opensans/v6/k3k702ZOKiLJc3WVjuplzHZ2MAKAc2x4R1uOSeegc5U.eot);
src: local('Open Sans Bold'), local('OpenSans-Bold'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/k3k702ZOKiLJc3WVjuplzHZ2MAKAc2x4R1uOSeegc5U.eot) format('embedded-opentype'), url(http://themes.googleusercontent.com/static/fonts/opensans/v6/k3k702ZOKiLJc3WVjuplzHhCUOGz7vYGh680lGh-uXM.woff) format('woff');
}
Regarding your second question: Web Fonts are not installed permanently on the client. They only live in the browser cache. If the cache is cleared, the font is downloaded again. Also, if two websites serve the same font from different URLs, the font is loaded twice.
But if the client OS happens to have a certain font installed system-wide, it will use the OS version without downloading. That's what the local
sources in the CSS are for.