HTTP Caching Explained: max-age, ETag and Why Your Users Still See Last Week's CSS

📺 Prefer to watch? 90-second YouTube Short · 💬 Telegram Originally published on software-engineer-blog.com . You fixed the CSS. You deployed. You opened the site and checked it yourself — perfect. Then a customer sends a screenshot of last week's layout. Nothing is broken. No deploy failed, no CDN is lying to you, no file is corrupt. The browser is doing exactly what you told it to do, several days ago, in a header you probably never wrote by hand. This is the part of web performance that gets skipped, because caching looks like a setting rather than a contract. It is a contract. And like any contract, the interesting part is not what it gives you — it is what you can no longer do once you have signed it. Throughout this post I will use one running example: PlantPal , a small plant shop. One stylesheet ( app.css ), one logo ( logo.png ), one API endpoint ( /api/products ). The floor: a page load is not one thing Before caching means anything, you have to see what it is acting on. Loading PlantPal's homepage is not a request. It is roughly 40 separate requests — the HTML, the stylesheet, a few fonts, the logo, a dozen product images, the JavaScript bundle, the product API. Each one is a full round trip: DNS is probably warm, but you still pay connection setup, the request, the server's think time, and the bytes coming back. The numbers for a first visit: ~40 requests 1.2 MB transferred 2.1 s to a usable page Which gives us the only sentence in this post that you actually need to remember: The fastest request is the one the browser never sends. Not a faster server. Not a closer edge node. Not a smaller file. No request at all. Everything below is a way of getting closer to that. max-age: buying silence The blunt instrument is Cache-Control : HTTP / 1.1 200 OK Content-Type : text/css Cache-Control : max-age=31536000 31536000 is one year in seconds. You are telling every browser that receives this response: keep this copy and use it for a year without asking me again. O