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When the form field gains focus:
Tell screen reader users that the input field is an auto-suggest.
Tell screen reader users they can select from the suggestion list or type their own input.
Tell screen reader users the number of suggestions that are available
Allow screen reader users to scroll through the list of suggestions by using the arrow keys.
When suggestions are arrowed through they are spoken aloud.
Add ARIA Labels for Screen Reader Users
Add role=”combobox” to the <input> field to tell screen reader users that the input field is an auto-suggest.
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https://codepen.io/ademcifcioglu/pen/xdOyXv
Error and Alert Messaging Within Forms
Please see Alert and Error Message Guidelines in connection with forms.