This page is about the title attribute, not the <title>-element.

Use title-attributes

In frames and iframes for a descriptive

Don't use title-attributes

On all other elements.

Relying on the title attribute is currently discouraged as many user agents do not expose the attribute in an accessible manner as required by this specification (e.g., requiring a pointing device such as a mouse to cause a tooltip to appear, which excludes keyboard-only users and touch-only users, such as anyone with a modern phone or tablet).

Source: HTML 5.2: The title attribute