Placeholders should only be used to show an example of the type of data that should be entered into a form; never as a replacement for a element; doing so harms accessibility and user experience.
The text is visually and programmatically associated with its corresponding form control. Screen readers don't announce placeholder content by default, but they do announce label content; it's the label that informs assistive technology users what data should be entered in the control. Labels also improve user experience for users of pointing devices: When a user clicks, touches, or taps a , focus is moved to the label's associated form control.
Placeholders can not be relied upon as a replacement for a label even for those not relying on assistive technology. Placeholder text is displayed at lower color contrast than the default form control text. This is by design, as you don't want users to be confused by what is placeholder text versus what is a filled-in form field. However, this lack of contrast can cause issues for low-vision users. Additionally, placeholder text disappears from form fields when users start entering text. If the placeholder text contains instructional information or examples that disappear, it can be confusing to users with cognitive issues and can make the form inaccessible if the placeholder contained the label.