In your menu bar, go up to "Show Emoji & Symbols." On the left-hand side, you should see a list of categories of symbols. You should see one called "Digits - All." If you don't, in the upper left corner, you should see a gear with an arrow. Click on it and select "Customize List." Add it to your list of character groups. Then open that group, and scroll down. You should see a bunch of fractions. Double-click the one you want to use.
½ ⅓ ⅔ ¼ ¾
Alternatively, if you don't want to go through all that rigamarole, just type the fraction. Safari should automatically change it from regular type.