+Cloning using `git` or downloading Github branches as zip files requires additional steps:
+
+ * Install [Composer](Unit-tests.md#install_composer) to manage third-party [PHP dependencies](3rd-party-libraries.md#composer).
+ * Install [yarn](https://yarnpkg.com/lang/en/docs/install/) to build the frontend dependencies.
+ * Install [python3-virtualenv](https://pypi.python.org/pypi/virtualenv) to build the local HTML documentation.
+