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WCAG: 1.1.1: Image buttons must have alternate text
<input type="image">elements have alternate text.
<input type="image">button must have alternate text, otherwise screen reader users will not know the button's purpose. Even if the image contains only text, it still requires alternate text, since a screen reader cannot translate images of words into output.
Text Alone Is Not A Label: Just typing text next to the form element is not sufficient to create a true label. Assistive technologies like screen readers require labels in code that can be determined programmatically. Some screen readers are programmed to guess what the label should be, based on the surrounding text, but this method is not fool-proof and can lead to confusion if the screen reader guesses wrong.