A useful new feature in the latest version of Safari for macOS High Sierra is the ability to set permanent zoom for any website. If a site has fonts that are too small, or too large, you can change them, and ensure that every time you visit the site, the change will be remembered. Safari does this automatically, but you can control the zoom from the app’s preferences.
Start by going to a website where you would like to change the font size. Open Safari’s preferences (choose Safari > Preferences), then click Websites. You’ll see a number of website options, including Page Zoom. Click that entry in the sidebar to see the websites that are open, and those that you have configured (if you’ve already changed the zoom).
Is you can see in the right-hand pane, the top section shows those sites that are currently open in Safari, and the bottom section shows Configured Websites, those where I have changed the zoom. All you need to do to add a side to the latter section is zoom one of its pages. To do this, press Command-Plus (+) or Command-Minus (-). The first time you zoom, the page will go to 115%, then to 125%, and so on.
Once you have zoomed a page, Safari will remember that zoom, adding that domain to the Configured Websites section. The next time you visit the site, it will display with the zoom you set.
You can also manually change the zoom settings for any site, either in the Currently Open Websites section or the Configured Websites section. You may want to change some sites so they display smaller or larger fonts, and rather than zoom from the keyboard, you can choose a zoom from one of the Currently Open Websites’ popup menus.