Final answer:
Using the 'required' attribute in an amount field of a form ensures users cannot submit the form without entering a value, which can prevent incomplete submissions but may also lead to a poor user experience if not managed correctly.
Step-by-step explanation:
When using the required attribute in the amount field in an HTML form, the primary side-effect is that the user will not be able to submit the form without entering a value in the amount field. This is because the required attribute specifies that a field must be filled out before submitting the form.
If users attempt to submit the form without filling out the amount field, they will receive a default browser error message, prompting them to fill in the missing information.
However, this can sometimes lead to a poor user experience, especially if the error message is not clear or if the field is not essential for every submission.