Test component extensively in screen readers to ensure there are no new issues created by use of shadow DOM

[annepetersen]

Test the new Web Component-based Banner component extensively in screen readers to ensure there are no new issues created by the use of shadow DOM

[annepetersen]

You reminded me of this today, @iamjolly. Going to open a separate issue about the accessibility testing guidance over in https://github.com/uswds/uswds-site

Edit: done