Quality values (quality values / priority) are numerical values used in HTTP headers to determine the relative quality or priority of response variants. In HTTP, they are often used in headers such as Accept and Content-Type. Quality values range from 0 to 1, where 1 denotes the highest quality (priority) and 0 the lowest.

For example, in the Accept header, different media types can be specified along with their quality values to indicate which types the client prioritizes more. For example, Accept: text/html;q=0.9, application/json;q=0.8 shows that the client is more inclined to accept HTML than JSON, as the quality value for HTML (0.9) is higher than for JSON (0.8).

Quality values allow establishing the order of priorities in the list of options and help clients and servers interact by selecting the most suitable response variants based on their conditions.