Simple header

A simple header (or CORS-safelisted request header) is one of the following HTTP headers:

• Accept,
• Accept-Language,
• Content-Language,
• Content-Type with a MIME type of its parsed value (ignoring parameters) of either application/x-www-form-urlencoded, multipart/form-data, or text/plain.

Or one of these client hint headers:

• DPR
• Downlink
• Save-Data
• Viewport-Width
• Width

When containing only simple headers, a requests is deemed simple and doesn't need to send a preflight request in the context of CORS.

Learn more

• HTTP headers
• Simple response header
• Forbidden header name
• Request header