Why Do Some CSS Links Contain a \'?\' Followed by a String of Characters?

Understanding the "?" in CSS Links

When exploring the HTML markup of a website, it's not uncommon to encounter links to CSS files with a mysterious "?" followed by a string of characters, as seen in this example:

<link href="/Content/all.min.css?d=20090107" rel="stylesheet" type="text/css" />

What's the Purpose of "?"

That mysterious "?" is a parameter that serves a specific purpose: ensuring the browser reloads a new version of the CSS file when changes are made.

When the server sends a CSS file to the browser, the browser usually caches it for future use. This caching improves performance by reducing subsequent requests for the same file. However, when the CSS file is updated, the cached version can prevent the browser from loading the latest changes.

Unique Filename Solution

The "?d=20090107" parameter adds a unique identifier to the filename. By changing the value after the "?", the web server ensures that the browser fetches a new copy of the CSS file, disregarding the cached version.

The web server typically ignores the parameter and serves the CSS file normally, so the content of the file remains unchanged. However, the browser treats the filename as a new resource and downloads the updated CSS file.

Noteworthy Considerations

• While dynamic CSS generation is a possibility, the use of a date parameter is often a simpler method for ensuring reload.
• As mentioned in Podcast 38, build processes can automate the tagging of CSS files with unique identifiers, eliminating the need for manual filename changes.

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