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Asked: 2026-05-13T09:54:25+00:00

This is a dangerously easy thing I feel I should know more about –

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This is a dangerously easy thing I feel I should know more about – but I don’t, and can’t find much around.

The question is: How exactly does a browser know a web page has changed?

Intuitively I would say that F5 refreshes the cache for a given page, and that cache is used for history navigation only and has an expiration date – which leads me to think the browser never knows if a web page has changed, and it just reloads the page if the cache is gone — but I am sure this is not always the case.

Any pointers appreciated!

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1 Answer

  1. Editorial Team

    Browsers will usually get this information through HTTP headers sent with the page.

    For example, the Last-Modified header tells the browser how old the page is. A browser can send a simple HEAD request to the page to get the last-modified value. If it’s newer than what the browser has in cache, then the browser can reload it.

    There are a bunch of other headers related to caching as well (like Cache-Control). Check out: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

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