Readable is a Web Extension that converts "custom font" text on webpages to standard characters, which can be read as intended by screen readers.
The Readable homepage has example text for comparison.
Install the add on from the Firefox Add On site.
The extension is undergoing review in the macOS App Store.
This Shortcut can be used to convert the contents of a page once on iOS Safari.
Install the add on from the Chrome Web Store.