Accessibility

Use this checklist below to ensure an acceptable level of accessibility.

Miscellaneous

Use open web standards such as HTML, CSS, JavaScript
Vet HTML and CSS using their respective W3C validators (HTML/CSS)
Include visually-hidden skip navigation that is operable via keyboard focus
Focus styles should be present and visible for keyboard operability
Images should always have an alt attribute , where its value conveys the same thing that is communicated to a sighted user. Decorative images that provide no important meaning to non-sighted users should utilize an empty alt="" value. This ensures screen readers omit them from announcements.
Timed responses are properly indicated to users.
All form fields can be activated, operated and submitted using a keyboard.
Users can control galleries and modal windows using a keyboard.

Use informed markup. Use informed labeling for the navigation itself along with current nav items. Labeling could be done via visually-hidden text or WAI-ARIA labels.
Navigations should have proper states (active, hover, focus, current, visited).
Dropdown navigations should be mouse/touch/keyboard operable and should leverage WAI-ARIA markup.

Inputs always have associated labels
Inputs should use an associated type attribute (e.g. search, tel, email, etc)
Make sure required fields are visually indicated as such (e.g. an asterisk *)
All form fields can be operated using a keyboard, with apparent focus styles.
Validation messages show all errors at once instead of progressively showing more errors
Validation messages use same field names as form and in the same order

Color combinations should be in compliance with WCAG 2.0 minimum contrast. Use a color contrast checker to ensure your ratio meets the standard.

Important information should not be conveyed with color alone. Use other visual identifiers like font-weight, sizing, shapes, special characters.