On a scrolling-enabled trackpad, scrolling up or down while pressing on the Command (Apple) key reduces/enlarges the fonts on your web browser. Happy scrolling!
First, you need to have a model with a scrolling trackpad. Second, you need to enable trackpad scrolling from System Preferences- Keyboard & Mouse.
...or (in Safari) you could go to View / Customize Address Bar and drag the Text Size button into the Address Bar.