Question: In a Timed Text Format for Transparent end-to-end Packet switched Streaming Service (PSS), what is the primary purpose of the 'alt' string in a HyperText link ('href')?
Option 1: To provide an alternative URL in case the primary URL is unreachable.
Option 2: To serve as a tool-tip or visual clue for user display, hinting at the linked URL's destination.
Option 3: To specify the character encoding (e.g., UTF-8) for the HyperText link.
Option 4: To define the styling (e.g., underlined blue text) of the visually indicated HyperText link.
Explanation: The 'alt' string is designed to offer a substitute for the URL, providing a hint to the user about the link's destination. This can be particularly useful in scenarios where displaying the full URL might not be feasible or desirable, thus enhancing user experience by giving a contextual clue without needing to access the linked content directly.