How to dynamically adjust font size

I would like to support many different browser window sizes, and by that I have figured out I need to shrink the font size for people with a more narrow window. For instance, I want such that people width a window size between 0-800 px must have font size x, between 800-1200 must have font size y, etc.
I have found a bmi calculator website that does this, which was discussed at stackoverflow. They mention, that you can accomplish this using either Javascript or CSS. But using WordPress, which method do you think is easiest for me to look into? I have only very limit experience using Javascript, and really no knowledge of CSS.
Does WordPress support both approaches or do they have an even easier solution?