A Reasonably Accessible Form

Complete and submit the form for a nice message.

Tips:

Placeholder text isn't a label. It should give an example of what input you should provide.
You need an actual visible label that is tied to the field it is labelling.
Indicate your required fields, both visually and with HTML attributes.
Make sure your tab order flows properly.
Make sure you have good focus indication.
Hitting 'Enter' should submit your form.
Group related fields (like radio buttons and checkboxes) together with fieldsets and legends
Use ARIA roles and attributes if you are doing anything beyond what semantic HTML supports. This form uses the following ARIA roles and attributes:
- aria-invalid and aria-describedby on the invalid inputs.
- role="alert" on the validation errors.

Things I didn't include:

Grouping of errors at the top of the form, with links to focus on the exact inputs.
Validation updating on blur or change; you have to resubmit the form to see a change in the errors.
Allow submitting the form without scripting. You can do this via the form's action attribute.

For a pretty darn awesome fully-accessible form, see: https://www.tenon-ui.info/forms-full-demo