What's the tabindex="-1" in bootstrap for

The tabindex attribute explicitly defines the navigation order for focusable elements (typically links and form controls) within a page. It can also be used to define whether elements should be focusable or not.

[Both] tabindex="0" and tabindex="-1" have special meaning and provide distinct functionality in HTML. A value of 0 indicates that the element should be placed in the default navigation order. This allows elements that are not natively focusable (such as <div>, <span>, and <p>) to receive keyboard focus. Of course one should generally use links and form controls for all interactive elements, but this does allow other elements to be focusable and trigger interaction.

A tabindex="-1" value removes the element from the default navigation flow (i.e., a user cannot tab to it), but it allows it to receive programmatic focus, meaning focus can be set to it from a link or with scripting.** This can be very useful for elements that should not be tabbed to, but that may need to have focus set to them.

A good example is a modal dialog window - when opened, focus should be set to the dialog so a screen reader will begin reading and the keyboard will begin navigating within the dialog. Because the dialog (probably just a <div> element) is not focusable by default, assigning it tabindex="-1" allows focus to be set to it with scripting when it is presented.

A value of -1 can also be useful in complex widgets and menus that utilize arrow keys or other shortcut keys to ensure that only one element within the widget is navigable with the tab key, but still allow focus to be set on other components within the widget.

Source: http://webaim.org/techniques/keyboard/tabindex

This is why you need tabindex="-1" on the modal <div>, so users can access common mouse and keyboard shortcuts. Hope that helps.

The tabindex attribute is associated with HTML, it is not specific to Bootstrap.

This property is responsible for indicating if an element is reachable by keyboard navigation.

You need to look out for three different scenarios:

1. When adding tabindex = "0" to an element this means it is reachable by the keyboard navigation but the order is defined by the documents source order.

2. When adding a positive value to the tabindex attribute (for example tabindex = "1", tabindex = "2") those elements are reachable by keyboard navigation and the order is defined by the value of the attribute.

3. When adding a negative value to tabindex (usually tabindex="-1") it means the element is not reachable by the keyboard navigation, but can be focused using focus function in JS.