Once the `WebDriver:GetElementShadowRoot` command is available in Marionette the appropriate support needs to be added to geckodriver.
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Once the `WebDriver:GetElementShadowRoot` command is available in Marionette the appropriate support needs to be added to geckodriver. Tests that need to pass can be found in the Wdspec test suite: https://searchfox.org/mozilla-central/source/testing/web-platform/tests/webdriver/tests/get_element_shadow_root