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 are wildcards ranked against specific types?

More specific values win ties: text/html beats text/* which beats */*. When q-values differ the higher q wins regardless of specificity, so text/plain;q=0.9 is preferred over text/html;q=0.8.

What does Accept-Encoding negotiate?

It lists the content-codings the client can decode, such as gzip, br (Brotli), deflate and zstd. identity means no encoding. The server compresses with the highest-q supported coding and echoes its choice in Content-Encoding.

Is Accept-Charset still used?

Largely deprecated. Modern clients omit it because UTF-8 is assumed everywhere, and most browsers no longer send it. It is included here for completeness and legacy systems.

Does this tool send my header anywhere?

No. Parsing happens entirely in your browser with JavaScript. Nothing is uploaded, logged or stored.

What is the HTTP Accept Headers Reference?

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. It runs free in your browser on Gera Tools, with nothing uploaded.

HTTP Accept Headers Reference

Name: HTTP Accept Headers Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

The four header families

Header	Negotiates	Server responds with
`Accept`	MIME type / media type	`Content-Type`
`Accept-Encoding`	Content coding (gzip, br, zstd)	`Content-Encoding`
`Accept-Language`	Natural language / locale	`Content-Language`
`Accept-Charset`	Character set (deprecated)	`Content-Type; charset=`

How q-values work

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 beats text/* beats */*). 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.

Parsing example

Given this Accept header:

Accept: text/html;q=0.9, application/json, */*;q=0.1

The parser ranks:

application/json — q=1.0 (default), specific type
text/html — q=0.9
*/* — q=0.1

A server that can serve both JSON and HTML will choose JSON because it carries the higher q. If the server can only serve HTML, it falls back to that.

Wildcard precedence rule

When two values have the same q, the more specific one wins: text/html (specific) > text/* (subtype wildcard) > */* (full wildcard). This matters when building middleware: a */*;q=1.0 in a client header does not automatically make every type equally preferred — an explicit text/html;q=0.8 still outranks the wildcard’s 0.8 because of its specificity.

Practical notes for API developers

Default q is 1; only add ;q= when you need a non-default preference — an explicit q=1.0 is harmless but adds noise.
q=0 is a hard reject — useful for opting out of a specific encoding (for example, gzip;q=0 to prevent gzip even when a wildcard * is present).
Any response that varies by an Accept-family header must include a Vary header listing it, or caches will serve the wrong variant to different clients.
Accept-Charset is effectively dead: assume UTF-8 everywhere and skip it for new APIs.