<label for="textBox1">Input</label>
<input id="textBox1" type="text" name="Text Box" required>

还是这样？

<label for="textBox1">Input</label>
<input id="textBox1" type="text" name="Text Box" aria-required="true">

还是这样？

<label for="textBox1">Input</label>
<input id="textBox1" type="text" name="Text Box" aria-required="true" required>

文章Accessible HTML5 Forms – Required Inputs声称最好实现两者.