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1. The font-size unit em is a relative measure: what is one em?
2. You can change the font-family of certain elements using CSS. You've seen us use the fonts Verdana, Courier, and Garamond. But how many fonts does CSS know? What are they?
One em is equal to the default font size on whatever screen the user is using. That makes it great for smartphone screens, since it doesn't try to tell the smartphone exactly how big to make a font: it just says, "Hey, 1em is the font size that you normally use, so 2em is twice as big and 0.5em is half that size!"
2. The answer is: it depends. Most computers will understand popular fonts like Verdana, Courier, and Garamond, but each individual computer has different fonts installed on it. The good news is that CSS has some built-in defaults meant to ensure your users see what you intend. They are:
serif: A font with little decorative bits on the ends of the strokes that make up letters. Do a search on "serif" to see what we mean.
sans-serif: A plain-looking font, like this one. It doesn't have the little doohickies on the ends of letters like a serif font does.
cursive: A scripty font! It looks like cursive writing.