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docs/configuring-playbook-client-hydrogen.md

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+# Configuring Hydrogen (optional)
+
+This playbook can install the [Hydrogen](https://github.com/vector-im/hydrogen-web) Matrix web client for you.
+Hydrogen is a lightweight web client that supports mobile and legacy web browsers.
+Hydrogen can be installed alongside or instead of Element.
+
+If you'd like Hydrogen to be installed, add the following to your configuration file (`inventory/host_vars/matrix.<your-domain>/vars.yml`):
+
+```yaml
+matrix_client_hydrogen_enabled: true
+```
+
+You will also need to add a DNS record so that Hydrogen can be accessed. 
+By default Hydrogen will use https://hydrogen.DOMAIN so you will need to create an CNAME record
+for `hydrogen`. See [docs/configuring-dns.md](docs/configuring-dns.md).
+
+If you would like to use a different domain, add the following to your configuration file (changing it to use your preferred domain):
+
+```yaml
+ matrix_server_fqn_hydrogen: "helium.{{ matrix_domain }}"
+```