Web browsers have caches, in which they keep pages after loading them,
so that if a page is revisited, it can be loaded from the local server
rather than from the remote server. This is a perfect system if pages
never change. If pages do change, there is a problem: the page might
change after a copy has been cached somewhere, so the user is presented
with an old version. We can have a fresh version by pressing ctrl + f5
key or by deleting cookies/history of from the browser settings.