What is ETags and conditional requests, and when does the browser use them?

An ETag (Entity Tag) is an opaque identifier the server assigns to a specific version of a resource.

// Server response
HTTP/1.1 200 OK
ETag: "abc123"
Cache-Control: no-cache
Content-Type: text/html

Revalidation flow:

1. Browser stores the response with its ETag.
2. On next request (after cache expires or with no-cache), browser sends: If-None-Match: "abc123"
3. If content unchanged → server responds 304 Not Modified with no body — saves bandwidth.
4. If content changed → server responds 200 OK with new content and new ETag.

Last-Modified / If-Modified-Since — older alternative using a timestamp instead of a hash. Less reliable (timestamps can be off).

When browsers use conditional requests:

• When Cache-Control: no-cache is set (revalidate every time).
• When max-age has expired.
• When the user does a soft reload (F5 on Windows).