ladybird/Libraries/LibWeb/Fetch/Headers.idl

// https://fetch.spec.whatwg.org/#typedefdef-headersinit
typedef (sequence<sequence<ByteString>> or record<ByteString, ByteString>) HeadersInit;

// https://fetch.spec.whatwg.org/#headers
[Exposed=(Window,Worker)]
interface Headers {
    constructor(optional HeadersInit init);

    undefined append(ByteString name, ByteString value);
    undefined delete(ByteString name);
    ByteString? get(ByteString name);
    sequence<ByteString> getSetCookie();
    boolean has(ByteString name);
    undefined set(ByteString name, ByteString value);
    iterable<ByteString, ByteString>;
};