Cache-Control tells browsers and CDNs how long they may reuse a downloaded resource before checking back with the server. A good policy means repeat visitors load pages from local cache instead of re-downloading everything, which dramatically speeds up returning visits and reduces server load.

It depends on the resource. Static assets that include a version hash in their filename (like main.a1b2c3.js) can use a very long max-age such as 31536000 (one year) with 'immutable'. HTML documents should use short or no caching so users always get fresh content. This tool flags both missing and overly short caching.

Expires is the older HTTP/1.0 way to set an absolute expiry date. Cache-Control is the modern HTTP/1.1 directive and uses a relative max-age in seconds. When both are present, Cache-Control takes precedence. You only need Expires for compatibility with very old caches; Cache-Control is what matters today.

Both are validators that let the browser ask 'has this changed?' without re-downloading the whole resource. ETag is a content fingerprint; Last-Modified is a timestamp. When a cached copy expires, the browser sends these back and the server can reply 304 Not Modified, saving bandwidth. Having at least one validator is good practice.

'no-store' forbids caching entirely, so the resource is fetched fresh every time, which is correct for sensitive data. 'no-cache' allows storing a copy but requires revalidation with the server before each reuse. They are commonly confused; using 'no-store' on static assets needlessly destroys performance.

Yes, indirectly but meaningfully. Strong caching improves repeat-visit load speed and Time to First Byte, which feed Core Web Vitals. It also lets Googlebot use conditional requests (304 responses), making crawling more efficient. Pages with no caching headers force full re-downloads on every visit, hurting perceived speed.

That is often intentional and correct. HTML usually changes when content updates, so caching it long-term would serve stale pages. A common pattern is no-cache or a very short max-age for HTML, paired with long max-age plus immutable for versioned CSS, JS, and images. This tool tells you which headers are present so you can confirm the policy fits the resource type.