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You can place a notice at the top of your document. This is good for when you have a document where everything is required to be filled out.
If only certain fields are required, mark them with an asterisk or the work required in parentheses within the visible field label. Include an explanatory note in the form instructions, such as: "All fields with an asterisk are required." Then, for screen reader users, you would include the word (Required) in the tooltip.
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