When you design forms for a wide audience, you should consider that some users may have disabilities that affect how they interact with the form. Some users may have vision impairment, reduced mobility, or other disabilities. To accommodate the needs of all users, you may find it worthwhile to include certain practical design features in your forms and to test forms using various assistive technologies.
A form is not accessible if it is not easy to use. You should try to design forms that are simple and usable. A simple layout of controls and fields with clear, meaningful captions and tool tips will make the form much easier for all users to fill.
Designing forms that are uncluttered and logically arranged, and that provide clear and simple instructions, will help all users to fill forms as easily as possible. Navigation features, such as the tabbing order and keyboard shortcuts, should support the logical order of objects on the form. The caption for a field should be on the same line as the field’s fillable area. Place captions consistently on the same side of the fields.
Choose objects that make the form easy to use. For example, when used properly, tables are an effective way to organize and present tabular information. Avoid overly complex tables, such as those with nested tables and sections.
Users should be able to fill the form completely using only the keyboard or an equivalent input device. Users with reduced mobility or impaired vision may have no choice but to use the keyboard, and many users who can use a mouse simply prefer keyboard input. By allowing various input methods, you not only create accessible forms, you also create forms that are better suited to the preferences of all users.
Design your form to interact normally with other applications and system standards. For example, you could support standard Windows Control Panel settings for colors and use standard keyboard behavior.
A well-designed form will be compatible with various assistive technologies. You should familiarize yourself with how these technologies work and use design techniques and user interface elements that are compatible with a wide range of assistive technologies.
Color can greatly enhance a user experience with your form but only if it used properly. Colors can emphasize and enhance certain parts of your form, but you should not convey information by color alone. Large amounts of color may cause eye pain. Too much color can obscure foreground text. High-contrast colors, such as the default settings of black on a white background, are recommended.
Images may help improve comprehension for users with some types of disabilities, However, many screen readers do not read graphics, which may decrease the accessibility of your form for users with vision impairments. If you choose to use images, provide text descriptions that describe the object and its purpose on the form.
Be aware that client-side scripts can interfere with screen readers and keyboards if the script changes the focus of the client application. For example, the change and mouseEnter events, when used with drop-down lists or list boxes, have the potential to cause inappropriate actions. Client-side scripting should be written to avoid problems with screen readers and keyboards. Similarly, avoid scripting events that cause visual effects, such as blinking text, which may increase readability issues for users.
Radio buttons are often misinterpreted, or difficult for users with disabilities to access on HTML forms because web browsers and screen readers treat them inconsistently. Avoid using radio buttons for HTML forms if you can use a list box instead.
See also 

General considerations for designing accessible forms