Slide1-4 Introduction To Screen Readers

Everyone that is browsing the web has a user agent. This is the software that acts as the bridge between the user and the internet. When your browser loads a web page, it identifies itself as an agent when it retrieves the content you have requested, and the browser sends a host of information about the device and network that you are on.
That is, the browser must understand the Application Protocol Interface (API) building blocks and command structure of the web page, so as to display meaningful information on the user device and allow the user to interact with the page elements.

There are many different browsers, each using a specific API structure and thus delivering a different user experience on different devices. However, for the most part, the browser API is designed for visual users and Mouse interaction, but some browsers have accommodated blind users by providing an accessibility interface structure for screen readers.

TPG: A Tale of Two Rooms - Understanding screen reader navigation