

To increase or decrease the zoom level of both text and images when viewing web pages in Safari, press Command and the + (plus) or - (minus) keys. You can also click on View in Safari's menu bar and select Zoom In or Zoom Out.
Alternatively, you can add zoom buttons to Safari's interface: Right-click (or Ctrl-click) on a space in the Safari toolbar and click Customize Toolbar.... Then drag the Zoom buttons in the dropdown to the space you just clicked on the toolbar. Click Done to finish.

If you want to keep images at the same size and only adjust web page font size on the fly, press Option-Command and the + or - keys. You can also hold down the Option key and click on View in the Safari menu bar, which changes the Zoom options to Make Text Bigger and Make Text Smaller.
Safari will remember your zoom and font size settings until you clear your History. To do so, click Safari in the menu bar, select Clear History..., then click the Clear History button.
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Article Link: How to Make Web Pages in Safari for Mac Easier to Read