The accessibility of any website starts with development and the firm you choose. It's essential to work with someone that has a strong understanding of web standards. If your current firm or agency isn't adhering to web standards, your users could be viewing a website that you didn’t intend them to see. Details like cross-browser compatibility and heading structure can ensure that your site displays correctly for your users and conveys your intentions to search engines.
I always use proper semantic markup and code to make sure the search engines index a site exactly as its designed. Accessible website development can be a simple process but the many steps are often overlooked by other firms.