CORS-safelisted request header - MDN Web Docs Glossary: Definitions of Web-related terms

A CORS-safelisted request header is one of the following HTTP headers:

When containing only these headers (and values that meet the additional requirements laid out below), a requests doesn't need to send a preflight request in the context of CORS.

You can safelist more headers using the Access-Control-Allow-Headers header and also list the above headers there to circumvent the following additional restrictions:

Additional restrictions

CORS-safelisted headers must also fulfill the following requirements in order to be a CORS-safelisted request header:

For Accept-Language and Content-Language: can only have values consisting of 0-9, A-Z, a-z, space or *,-.;=.
For Accept and Content-Type: can't contain a CORS-unsafe request header byte: 0x00-0x1F (except 0x09 (HT)), "():<>?@[\]{}, and 0x7F (DEL).
For Content-Type: needs to have a MIME type of its parsed value (ignoring parameters) of either application/x-www-form-urlencoded, multipart/form-data, or text/plain.
For any header: the value’s length can't be greater than 128.