You might be thinking of "autocomplete". You can find autocomplete information in the combo box design pattern in the WAI-ARIA Authoring Practices 1.1. The design pattern contains information for making the concept accessible.
However, what you describe is a little different from autocomplete, though. It sounds like you want the user to be able to start typing no matter where they are on the page and have the focus automatically move to the search field with that first letter entered. I would not recommend that. Having the focus move without the user's consent can be disorienting. You can provide a shortcut key to the search field if you want (such as the
accesskey property), but that too can have issues (such as the discoverability of the access key).
Screen reader users use single key shortcuts to navigate to certain web elements. For example, 'B' will take you to the next button, 'T' will take you to a table, 'L' will take you to a list, 'H' will take you to a header, etc. If you want a screen reader user to benefit from your "instant search", they'd either have to:
- switch out of navigation mode so that keyboard input goes to your application instead of the screen reader, OR
- temporarily send keystrokes to the application instead of the screen reader, OR
- be forced into "application mode" by your website, which automatically turns off the screen reader navigation mode.
None of these are very pleasant, and number 3 is the least desirable. Having a dedicated search field is usually the best way to go and allow the user to put the focus there themselves. (It you use
role='search' in your html, that will allow a screen reader user to quickly move to that area by using landmark navigation.)