The section reads
Textual information shall be provided through operating system functions for displaying text. The minimum information that shall be made available is text content, text input caret location, and text attributes.
Does anyone know what that means? Simply that the state of the text elements be reported to other applications via the OS? If so, does the browser take care of this automatically for a web app, or is there something that we have to do to support it?
Short Version
The browser will handle all of this for you assuming you adhere to web standards / WCAG.
Longer Version
The web browser will deal with the "Operating System Functions" part for you, as that will turn the HTML into the accessibility tree, which is exposed through system APIs for assistive technology.
With regards to the other parts:
aria-hidden="true"or do something equally unusual this will be handled automatically (for textual content).<input>,<textarea>. If you create a custom widget or userole="application"(don't userole="application"unless you really like pain!) this will be handled automatically for you.<strong>,<mark>etc. If you create custom text attributes you will need to use some WAI-ARIA to describe the text state.So basically unless you really go "outside of the box" with a component you will be fine.
Assuming you are following WCAG you will almost certainly automatically meet these criteria when developing a web application.