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.
