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HTTP Accept Headers Reference

Accept, Accept-Encoding, Accept-Language, Accept-Charset with q-value syntax.

Reference for HTTP content-negotiation Accept-family headers with q-factor weighting, wildcard rules and a live q-value preference parser for any Accept header string.

What is a q-value?

A q-value (quality factor) is a weight from 0 to 1 attached to a value with ;q=, expressing relative preference. q=1 is the default and most preferred; q=0 explicitly rejects a value. Servers pick the acceptable option with the highest q.

How clients say what they can accept

Proactive content negotiation lets a client tell the server which formats, compression schemes and languages it prefers, and the server picks the best match. This reference covers the four Accept-family headers, the q-value weighting syntax they share, and the wildcard precedence rules — plus a parser that sorts any header by preference.

How it works

Each header is a comma-separated list of values, each optionally carrying a quality factor with ;q=:

Accept: text/html, application/xhtml+xml, application/xml;q=0.9, */*;q=0.8
Accept-Encoding: br;q=1.0, gzip;q=0.8, *;q=0.1
Accept-Language: en-GB, en;q=0.7, fr;q=0.3

The server scores every value it can produce by the client’s q (default 1), breaking ties by specificity (text/html > text/* > */*). A value with q=0 is explicitly refused. The chosen variant is reflected back — in Content-Type, Content-Encoding or Content-Language — and the response should carry a matching Vary header so caches key correctly.

Tips and notes

  • Default q is 1; only add ;q= when you need a non-default preference.
  • q=0 is a hard reject, useful for opting out of a specific coding or language.
  • A response that varies by these headers must send Vary listing them.
  • Accept-Charset is effectively dead — assume UTF-8 and ignore it for new APIs.