Léonie Watson explains how the HTML tabindex attribute is used to manage keyboard focus. Of particular interest to me was a clarification of what tabindex="-1" does (because I always forget).
tabindex="-1"
The element becomes programmatically focusable but isn’t included in the tab order. It can’t be reached by someone using the tab key to navigate through content, but it can be focused on with scripting via the focus() method.
We might use this because we want to be able to set focus to a particular element via a script. Léonie offers the example of a list of form errors.
Alternatively we might use this because we want to prevent a normally tabbable element from being tabbable. Sara Souidean uses this technique (in combination with aria-hidden=true) on e-commerce product teaser “media objects” in order to limit the number of duplicate links that keyboard users must tab through.
tabindex="0"
Applying tabindex="0" to an element inserts it into the tabbing order based on its position in the source. It is not necessary to apply this to an interactive element such as a button or checkbox since they are focusable by default. You should use it sparingly (because it requires care to ensure accessibility) but you might use it on a custom element which needs to be interactive under certain circumstances such as the horizontally scrollable container in a Data Table.
tabindex="1+"
This imposes a tab order on the content that bears no resemblance to the expected tab order. It’s an antipattern. Don’t do it.