: This is the q-factor (quality value). It is a weight from 0.0 to 1.0 that tells the server how much the user prefers that specific language. Technical Role in "Content Negotiation"
: Properly configuring these headers is crucial for SEO and ensuring users see content in their preferred language without manual selection. 0.9,en-us
When you visit a website, your browser sends these values to help the server decide which version of a page to serve. If a server sees 0.9,en-us , it understands that US English is a high priority (90% preference) but perhaps second to another language set at 1.0 . : This is the q-factor (quality value)
: Security researchers (e.g., on HackerOne ) often include their full browser headers, containing these strings, when reporting vulnerabilities like SQL injections or XSS to show the exact environment used for the exploit. Mozilla Firefox 1.5.0.9 Release Notes When you visit a website, your browser sends
: Refers to the language code for English as used in the United States.
In a standard HTTP request, you might see a header like this: Accept-Language: fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5
: Developers often look for this string in "Network" tab logs when debugging why a site is displaying the "wrong" language or failing to load localized resources. Other Contexts