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+# Selenium atoms
+
+Marionette uses a small list of [Selenium atoms] to interact with
+web elements.  Initially those have been added to ensure a better
+reliability due to a wider usage inside the Selenium project. But
+by adding full support for the [WebDriver specification] they will
+be removed step by step.
+
+Currently the following atoms are in use:
+
+- `getElementText`
+- `isElementDisplayed`
+- `isElementEnabled`
+
+To use one of those atoms Javascript modules will have to import
+[atom.sys.mjs].
+
+[Selenium atoms]: https://github.com/SeleniumHQ/selenium/tree/master/javascript/webdriver/atoms
+[WebDriver specification]: https://w3c.github.io/webdriver/webdriver-spec.html
+[atom.sys.mjs]: https://searchfox.org/mozilla-central/source/remote/marionette/atom.sys.mjs
+
+## Update required Selenium atoms
+
+In regular intervals the atoms, which are still in use, have to
+be updated.  Therefore they have to be exported from the Selenium
+repository first, and then updated in [atom.sys.mjs].
+
+### Export Selenium Atoms
+
+The canonical GitHub repository for Selenium is
+
+  <https://github.com/SeleniumHQ/selenium.git>
+
+so make sure to have an up-to-date local copy of it. If you have to clone
+it first, it is recommended to specify the `--depth=1` argument, so only the
+last changeset is getting downloaded (which itself might already be
+more than 100 MB).
+
+```bash
+git clone --depth=1 https://github.com/SeleniumHQ/selenium.git
+```
+
+To export the correct version of the atoms identify the changeset id (SHA1) of
+the Selenium repository in the [index section] of the WebDriver specification.
+
+Fetch that changeset and check it out:
+
+```bash
+git fetch --depth=1 origin SHA1
+git checkout SHA1
+```
+
+Now you can export all the required atoms by running the following
+commands. Make sure to [install bazelisk] first.
+
+```bash
+bazel build //javascript/atoms/fragments:get-text
+bazel build //javascript/atoms/fragments:is-displayed
+bazel build //javascript/atoms/fragments:is-enabled
+```
+
+For each of the exported atoms a file can now be found in the folder
+`bazel-bin/javascript/atoms/fragments/`.  They contain all the
+code including dependencies for the atom wrapped into a single function.
+
+[index section]: <https://w3c.github.io/webdriver/#index>
+[install bazelisk]: <https://github.com/bazelbuild/bazelisk#installation>
+
+### Update atom.sys.mjs
+
+To update the atoms for Marionette the `atoms.js` file has to be edited. For
+each atom to be updated the steps as laid out below have to be performed:
+
+1. Open the Javascript file of the exported atom. See above for
+   its location.
+
+2. Add the related function name and `element` as parameters to the wrapper
+   function, which can be found at the very beginning of the file so that it
+   is equal to the parameters in `atom.sys.mjs`.
+
+3. Copy and paste the whole contents of the file into the left textarea on
+   <https://jsonformatter.org/json-stringify-online> to get a stringified
+   version of all the required functions.
+
+4. Copy and paste the whole contents of the right textarea, and replace the
+   existing code for the atom in `atom.sys.mjs`.
+
+### Test the changes
+
+To ensure that the update of the atoms doesn't cause a regression
+a try build should be run including Marionette unit tests, Firefox
+ui tests, and all the web-platform-tests.