Building accessible web content can start from the coding roots of a project by using semantic HTML, CSS, and Javascript. This semantic practice can be easily applied to web applications, particularly when using the WAI-ARIA as a guide. The following resources summarize semantic coding practices, including accessibility auditing tools for HTML, CSS, and Javascript.
Mozilla Developers Network HTML: A Good Basis for Accountability
Mozilla Developers Network's WAI-ARIA discusses building accessible web applications