Quality values

Quality values, or q-values or q-factors, are used to describe the order of priority of values in a comma-separated list. It is a special syntax allowed in some HTTP headers and in HTML. The importance of a value is marked by the suffix ';q=' immediately followed by a value between 0 and 1 included, with up to three decimal digits, the highest value denoting the highest priority. When not present, the default value is 1.

Examples

The following syntax

text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

indicates that the order of priority is this:

Value Priority
text/html and application/xhtml+xml 1.0
application/xml 0.9
*/* 0.8

There is no priority defined for the first two values, the order in the list is irrelevant. Nevertheless, with the same quality, more specific values have priority over less specific ones:

text/html;q=0.8,text/*;q=0.8,*/*;q=0.8
Value Priority
text/html 0.8 (but totally specified)
application/xml 0.8 (partially specified)
*/* 0.8 (not specified)

Some syntax, like the one of Accept allow additional specifiers like text/html;level=1. These increase the specificity of the value. Their use is extremely rare.

Browser-specific information

Firefox

Starting with Firefox 18, the quality factor values are clamped to 2 decimal places. They used to be clamped to only 1 decimal place in earlier versions (bugĀ 672448).

More information

Document Tags and Contributors

 Contributors to this page: fscholz, teoli
 Last updated by: fscholz,