why css don't load fonts-family

I do not know what I am doing wrong. I was learning how to make a basic html and the YouTuber was able to us the code:

font-family: Arial, Helvetica, sans-serif

I attempt to do the same, but “sans-serif” is the only one that works. Can someone explain to me how to fix this? I'm using sublime text 3 on windows 7

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