What are the validity directives？

Validity directives are instructions given to web browsers by web servers that specify how long a cached resource can be considered valid before it must be re-validated. These directives help to improve website performance by reducing the number of requests made to the server for resources that have not changed. The most common validity directives are:

1. Cache-Control: This directive specifies the caching behavior of a resource and can include values such as max-age, no-cache, and must-revalidate.

2. Expires: This directive specifies the date and time after which a cached resource is considered stale and must be re-validated.

3. ETag: This directive provides a unique identifier for a resource that can be used to determine if the cached copy is still valid.

4. Last-Modified: This directive specifies the date and time that a resource was last modified, which can be used to determine if the cached copy is still valid.

By using these validity directives, web developers can improve website performance and reduce server load, while still ensuring that users are always seeing the most up-to-date content.