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+# Font tests
+
+The font tests check if PDF.js can read font data correctly. For validation
+the `ttx` tool (from the Python `fonttools` library) is used that can convert
+font data to an XML format that we can easily use for assertions in the tests.
+In the font tests we let PDF.js read font data and pass the PDF.js-interpreted
+font data through `ttx` to check its correctness. The font tests are successful
+if PDF.js can successfully read the font data and `ttx` can successfully read
+the PDF.js-interpreted font data back, proving that PDF.js does not apply any
+transformations that break the font data.
+
+## Running the font tests
+
+The font tests are run on GitHub Actions using the workflow defined in
+`.github/workflows/font_tests.yml`, but it is also possible to run the font
+tests locally. The current stable versions of the following dependencies are
+required to be installed on the system:
+
+- Python 3
+- `fonttools` (see https://pypi.org/project/fonttools and https://github.com/fonttools/fonttools)
+
+The recommended way of installing `fonttools` is using `pip` in a virtual
+environment because it avoids having to do a system-wide installation and
+therefore improves isolation, but any other way of installing `fonttools`
+that makes `ttx` available in the `PATH` environment variable also works.
+
+Using the virtual environment approach the font tests can be run locally by
+creating and sourcing a virtual environment with `fonttools` installed in
+it before running the font tests:
+    
+```
+python3 -m venv venv
+source venv/bin/activate
+pip install fonttools
+npx gulp fonttest
+```