diff --git a/actix-web/src/http/header/accept.rs b/actix-web/src/http/header/accept.rs
index 99c95175f..db2938437 100644
--- a/actix-web/src/http/header/accept.rs
+++ b/actix-web/src/http/header/accept.rs
@@ -26,49 +26,49 @@ common_header! {
     /// accept-ext = OWS ";" OWS token [ "=" ( token / quoted-string ) ]
     /// ```
     ///
+    /// # Note
+    /// This is a request header. Servers should not send `Accept` in responses; to describe the
+    /// response body media type, use [`ContentType`](super::ContentType) / the `Content-Type`
+    /// header instead.
+    ///
     /// # Example Values
     /// * `audio/*; q=0.2, audio/basic`
     /// * `text/plain; q=0.5, text/html, text/x-dvi; q=0.8, text/x-c`
     ///
     /// # Examples
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::{Accept, QualityItem};
+    /// use actix_web::{http::header::{Accept, QualityItem}, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(
-    ///     Accept(vec![
-    ///         QualityItem::max(mime::TEXT_HTML),
-    ///     ])
-    /// );
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(Accept(vec![QualityItem::max(mime::TEXT_HTML)]))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     ///
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::{Accept, QualityItem};
+    /// use actix_web::{http::header::{Accept, QualityItem}, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(
-    ///     Accept(vec![
-    ///         QualityItem::max(mime::APPLICATION_JSON),
-    ///     ])
-    /// );
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(Accept(vec![QualityItem::max(mime::APPLICATION_JSON)]))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     ///
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::{Accept, QualityItem, q};
+    /// use actix_web::{http::header::{Accept, Header as _, QualityItem, q}, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(
-    ///     Accept(vec![
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(Accept(vec![
     ///         QualityItem::max(mime::TEXT_HTML),
     ///         QualityItem::max("application/xhtml+xml".parse().unwrap()),
     ///         QualityItem::new(mime::TEXT_XML, q(0.9)),
     ///         QualityItem::max("image/webp".parse().unwrap()),
     ///         QualityItem::new(mime::STAR_STAR, q(0.8)),
-    ///     ])
-    /// );
+    ///     ]))
+    ///     .to_http_request();
+    ///
+    /// let accept = Accept::parse(&req).unwrap();
+    /// assert_eq!(accept.preference(), mime::TEXT_HTML);
     /// ```
     ///
     /// [RFC 7231 §5.3.2]: https://datatracker.ietf.org/doc/html/rfc7231#section-5.3.2
diff --git a/actix-web/src/http/header/accept_charset.rs b/actix-web/src/http/header/accept_charset.rs
index 43a7861fe..04216f69a 100644
--- a/actix-web/src/http/header/accept_charset.rs
+++ b/actix-web/src/http/header/accept_charset.rs
@@ -10,6 +10,10 @@ common_header! {
     /// to an origin server that is capable of representing information in
     /// those charsets.
     ///
+    /// # Note
+    /// This is a request header. Servers should not send `Accept-Charset` in responses; to
+    /// describe the response body's charset, set an appropriate `Content-Type` header instead.
+    ///
     /// # ABNF
     /// ```plain
     /// Accept-Charset = 1#( ( charset / "*" ) [ weight ] )
@@ -20,36 +24,33 @@ common_header! {
     ///
     /// # Examples
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::{AcceptCharset, Charset, QualityItem};
+    /// use actix_web::{http::header::{AcceptCharset, Charset, QualityItem}, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(
-    ///     AcceptCharset(vec![QualityItem::max(Charset::Us_Ascii)])
-    /// );
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(AcceptCharset(vec![QualityItem::max(Charset::Us_Ascii)]))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     ///
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::{AcceptCharset, Charset, q, QualityItem};
+    /// use actix_web::{http::header::{AcceptCharset, Charset, q, QualityItem}, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(
-    ///     AcceptCharset(vec![
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(AcceptCharset(vec![
     ///         QualityItem::new(Charset::Us_Ascii, q(0.9)),
     ///         QualityItem::new(Charset::Iso_8859_10, q(0.2)),
-    ///     ])
-    /// );
+    ///     ]))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     ///
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::{AcceptCharset, Charset, QualityItem};
+    /// use actix_web::{http::header::{AcceptCharset, Charset, QualityItem}, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(
-    ///     AcceptCharset(vec![QualityItem::max(Charset::Ext("utf-8".to_owned()))])
-    /// );
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(AcceptCharset(vec![QualityItem::max(Charset::Ext("utf-8".to_owned()))]))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     ///
     /// [RFC 7231 §5.3.3]: https://datatracker.ietf.org/doc/html/rfc7231#section-5.3.3
diff --git a/actix-web/src/http/header/accept_encoding.rs b/actix-web/src/http/header/accept_encoding.rs
index 19d649926..ac85ec9e1 100644
--- a/actix-web/src/http/header/accept_encoding.rs
+++ b/actix-web/src/http/header/accept_encoding.rs
@@ -11,6 +11,11 @@ common_header! {
     /// content-codings are acceptable in the response. An `identity` token is used as a synonym
     /// for "no encoding" in order to communicate when no encoding is preferred.
     ///
+    /// # Note
+    /// This is a request header. Servers should not send `Accept-Encoding` in responses; use the
+    /// `Content-Encoding` header (or middleware like compression) to describe any content-coding
+    /// applied to the response body.
+    ///
     /// # ABNF
     /// ```plain
     /// Accept-Encoding  = #( codings [ weight ] )
@@ -26,26 +31,26 @@ common_header! {
     ///
     /// # Examples
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::{AcceptEncoding, Encoding, Preference, QualityItem};
+    /// use actix_web::{http::header::{AcceptEncoding, Encoding, Preference, QualityItem}, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(
-    ///     AcceptEncoding(vec![QualityItem::max(Preference::Specific(Encoding::gzip()))])
-    /// );
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(AcceptEncoding(vec![
+    ///         QualityItem::max(Preference::Specific(Encoding::gzip())),
+    ///     ]))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     ///
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::{AcceptEncoding, Encoding, QualityItem};
+    /// use actix_web::{http::header::{AcceptEncoding, QualityItem}, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(
-    ///     AcceptEncoding(vec![
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(AcceptEncoding(vec![
     ///         "gzip".parse().unwrap(),
     ///         "br".parse().unwrap(),
-    ///     ])
-    /// );
+    ///     ]))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     (AcceptEncoding, header::ACCEPT_ENCODING) => (QualityItem<Preference<Encoding>>)*
 
diff --git a/actix-web/src/http/header/accept_language.rs b/actix-web/src/http/header/accept_language.rs
index b1d588f8d..0f7800d16 100644
--- a/actix-web/src/http/header/accept_language.rs
+++ b/actix-web/src/http/header/accept_language.rs
@@ -14,6 +14,10 @@ common_header! {
     /// [RFC 7231 §5.3.5](https://datatracker.ietf.org/doc/html/rfc7231#section-5.3.5) using language
     /// ranges defined in [RFC 4647 §2.1](https://datatracker.ietf.org/doc/html/rfc4647#section-2.1).
     ///
+    /// # Note
+    /// This is a request header. Servers should not send `Accept-Language` in responses; use
+    /// `Content-Language` to describe the language of the response body.
+    ///
     /// # ABNF
     /// ```plain
     /// Accept-Language = 1#( language-range [ weight ] )
@@ -31,29 +35,25 @@ common_header! {
     ///
     /// # Examples
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::{AcceptLanguage, QualityItem};
+    /// use actix_web::{http::header::{AcceptLanguage, QualityItem}, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(
-    ///     AcceptLanguage(vec![
-    ///         "en-US".parse().unwrap(),
-    ///     ])
-    /// );
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(AcceptLanguage(vec!["en-US".parse().unwrap()]))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     ///
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::{AcceptLanguage, QualityItem, q};
+    /// use actix_web::{http::header::{AcceptLanguage, q, QualityItem}, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(
-    ///     AcceptLanguage(vec![
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(AcceptLanguage(vec![
     ///         "da".parse().unwrap(),
     ///         "en-GB;q=0.8".parse().unwrap(),
     ///         "en;q=0.7".parse().unwrap(),
-    ///     ])
-    /// );
+    ///     ]))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     (AcceptLanguage, header::ACCEPT_LANGUAGE) => (QualityItem<Preference<LanguageTag>>)*
 
diff --git a/actix-web/src/http/header/if_match.rs b/actix-web/src/http/header/if_match.rs
index e0b46a6c3..d6e1e2c4b 100644
--- a/actix-web/src/http/header/if_match.rs
+++ b/actix-web/src/http/header/if_match.rs
@@ -16,6 +16,11 @@ common_header! {
     /// intends this precondition to prevent the method from being applied if
     /// there have been any changes to the representation data.
     ///
+    /// # Note
+    /// This is a request header used for conditional requests (typically to avoid lost updates).
+    /// Servers should not send `If-Match` in responses; use [`ETag`](super::ETag) to describe the
+    /// current representation instead.
+    ///
     /// # ABNF
     /// ```plain
     /// If-Match = "*" / 1#entity-tag
@@ -27,25 +32,25 @@ common_header! {
     ///
     /// # Examples
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::IfMatch;
+    /// use actix_web::{http::header::IfMatch, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(IfMatch::Any);
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(IfMatch::Any)
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     ///
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::{IfMatch, EntityTag};
+    /// use actix_web::{http::header::{EntityTag, IfMatch}, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(
-    ///     IfMatch::Items(vec![
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(IfMatch::Items(vec![
     ///         EntityTag::new(false, "xyzzy".to_owned()),
     ///         EntityTag::new(false, "foobar".to_owned()),
     ///         EntityTag::new(false, "bazquux".to_owned()),
-    ///     ])
-    /// );
+    ///     ]))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     (IfMatch, IF_MATCH) => {Any / (EntityTag)+}
 
diff --git a/actix-web/src/http/header/if_modified_since.rs b/actix-web/src/http/header/if_modified_since.rs
index 8547ff490..9367db62d 100644
--- a/actix-web/src/http/header/if_modified_since.rs
+++ b/actix-web/src/http/header/if_modified_since.rs
@@ -10,9 +10,14 @@ crate::http::header::common_header! {
     /// Transfer of the selected representation's data is avoided if that
     /// data has not changed.
     ///
+    /// # Note
+    /// This is a request header used for cache validation. Servers should not send
+    /// `If-Modified-Since` in responses; use [`LastModified`](super::LastModified) / the
+    /// `Last-Modified` header instead.
+    ///
     /// # ABNF
     /// ```plain
-    /// If-Unmodified-Since = HTTP-date
+    /// If-Modified-Since = HTTP-date
     /// ```
     ///
     /// # Example Values
@@ -22,14 +27,13 @@ crate::http::header::common_header! {
     ///
     /// ```
     /// use std::time::{SystemTime, Duration};
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::IfModifiedSince;
+    /// use actix_web::{http::header::IfModifiedSince, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
     /// let modified = SystemTime::now() - Duration::from_secs(60 * 60 * 24);
-    /// builder.insert_header(
-    ///     IfModifiedSince(modified.into())
-    /// );
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(IfModifiedSince(modified.into()))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     (IfModifiedSince, IF_MODIFIED_SINCE) => [HttpDate]
 
diff --git a/actix-web/src/http/header/if_none_match.rs b/actix-web/src/http/header/if_none_match.rs
index 1a424df96..ed8bbde81 100644
--- a/actix-web/src/http/header/if_none_match.rs
+++ b/actix-web/src/http/header/if_none_match.rs
@@ -15,6 +15,11 @@ crate::http::header::common_header! {
     /// can be used for cache validation even if there have been changes to
     /// the representation data.
     ///
+    /// # Note
+    /// This is a request header used for cache validation (and conditional requests). Servers
+    /// should not send `If-None-Match` in responses; use [`ETag`](super::ETag) to describe the
+    /// current representation instead.
+    ///
     /// # ABNF
     /// ```plain
     /// If-None-Match = "*" / 1#entity-tag
@@ -29,25 +34,25 @@ crate::http::header::common_header! {
     ///
     /// # Examples
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::IfNoneMatch;
+    /// use actix_web::{http::header::IfNoneMatch, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(IfNoneMatch::Any);
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(IfNoneMatch::Any)
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     ///
     /// ```
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::{IfNoneMatch, EntityTag};
+    /// use actix_web::{http::header::{EntityTag, IfNoneMatch}, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
-    /// builder.insert_header(
-    ///     IfNoneMatch::Items(vec![
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(IfNoneMatch::Items(vec![
     ///         EntityTag::new(false, "xyzzy".to_owned()),
     ///         EntityTag::new(false, "foobar".to_owned()),
     ///         EntityTag::new(false, "bazquux".to_owned()),
-    ///     ])
-    /// );
+    ///     ]))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     (IfNoneMatch, IF_NONE_MATCH) => {Any / (EntityTag)+}
 
diff --git a/actix-web/src/http/header/if_range.rs b/actix-web/src/http/header/if_range.rs
index 3e8727ab0..d611a5897 100644
--- a/actix-web/src/http/header/if_range.rs
+++ b/actix-web/src/http/header/if_range.rs
@@ -22,6 +22,9 @@ use crate::{error::ParseError, http::header, HttpMessage};
 /// representation is unchanged, send me the part(s) that I am requesting
 /// in Range; otherwise, send me the entire representation.
 ///
+/// # Note
+/// This is a request header. Servers should not send `If-Range` in responses.
+///
 /// # ABNF
 /// ```plain
 /// If-Range = entity-tag / HTTP-date
@@ -34,26 +37,23 @@ use crate::{error::ParseError, http::header, HttpMessage};
 ///
 /// # Examples
 /// ```
-/// use actix_web::HttpResponse;
-/// use actix_web::http::header::{EntityTag, IfRange};
+/// use actix_web::{http::header::{EntityTag, IfRange}, test};
 ///
-/// let mut builder = HttpResponse::Ok();
-/// builder.insert_header(
-///     IfRange::EntityTag(
-///         EntityTag::new(false, "abc".to_owned())
-///     )
-/// );
+/// let req = test::TestRequest::default()
+///     .insert_header(IfRange::EntityTag(EntityTag::new(false, "abc".to_owned())))
+///     .to_http_request();
+/// # let _ = req;
 /// ```
 ///
 /// ```
 /// use std::time::{Duration, SystemTime};
-/// use actix_web::{http::header::IfRange, HttpResponse};
+/// use actix_web::{http::header::IfRange, test};
 ///
-/// let mut builder = HttpResponse::Ok();
 /// let fetched = SystemTime::now() - Duration::from_secs(60 * 60 * 24);
-/// builder.insert_header(
-///     IfRange::Date(fetched.into())
-/// );
+/// let req = test::TestRequest::default()
+///     .insert_header(IfRange::Date(fetched.into()))
+///     .to_http_request();
+/// # let _ = req;
 /// ```
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum IfRange {
diff --git a/actix-web/src/http/header/if_unmodified_since.rs b/actix-web/src/http/header/if_unmodified_since.rs
index afa4eb8e5..af32d7e46 100644
--- a/actix-web/src/http/header/if_unmodified_since.rs
+++ b/actix-web/src/http/header/if_unmodified_since.rs
@@ -10,6 +10,11 @@ crate::http::header::common_header! {
     /// This field accomplishes the same purpose as If-Match for cases where
     /// the user agent does not have an entity-tag for the representation.
     ///
+    /// # Note
+    /// This is a request header used for conditional requests. Servers should not send
+    /// `If-Unmodified-Since` in responses; use [`LastModified`](super::LastModified) / the
+    /// `Last-Modified` header instead.
+    ///
     /// # ABNF
     /// ```plain
     /// If-Unmodified-Since = HTTP-date
@@ -22,14 +27,13 @@ crate::http::header::common_header! {
     ///
     /// ```
     /// use std::time::{SystemTime, Duration};
-    /// use actix_web::HttpResponse;
-    /// use actix_web::http::header::IfUnmodifiedSince;
+    /// use actix_web::{http::header::IfUnmodifiedSince, test};
     ///
-    /// let mut builder = HttpResponse::Ok();
     /// let modified = SystemTime::now() - Duration::from_secs(60 * 60 * 24);
-    /// builder.insert_header(
-    ///     IfUnmodifiedSince(modified.into())
-    /// );
+    /// let req = test::TestRequest::default()
+    ///     .insert_header(IfUnmodifiedSince(modified.into()))
+    ///     .to_http_request();
+    /// # let _ = req;
     /// ```
     (IfUnmodifiedSince, IF_UNMODIFIED_SINCE) => [HttpDate]
 
diff --git a/actix-web/src/http/header/range.rs b/actix-web/src/http/header/range.rs
index 4a5d95d93..0f0958e5c 100644
--- a/actix-web/src/http/header/range.rs
+++ b/actix-web/src/http/header/range.rs
@@ -15,6 +15,10 @@ use super::{Header, HeaderName, HeaderValue, InvalidHeaderValue, TryIntoHeaderVa
 /// only one or more sub-ranges of the selected representation data, rather than the entire selected
 /// representation data.
 ///
+/// # Note
+/// This is a request header. Servers should not send `Range` in responses; use
+/// [`ContentRange`](super::ContentRange) / the `Content-Range` header for partial responses.
+///
 /// # ABNF
 /// ```plain
 /// Range = byte-ranges-specifier / other-ranges-specifier
@@ -42,16 +46,18 @@ use super::{Header, HeaderName, HeaderValue, InvalidHeaderValue, TryIntoHeaderVa
 ///
 /// # Examples
 /// ```
-/// use actix_web::http::header::{Range, ByteRangeSpec};
-/// use actix_web::HttpResponse;
+/// use actix_web::{http::header::{ByteRangeSpec, Range}, test};
 ///
-/// let mut builder = HttpResponse::Ok();
-/// builder.insert_header(Range::Bytes(
-///     vec![ByteRangeSpec::FromTo(1, 100), ByteRangeSpec::From(200)]
-/// ));
-/// builder.insert_header(Range::Unregistered("letters".to_owned(), "a-f".to_owned()));
-/// builder.insert_header(Range::bytes(1, 100));
-/// builder.insert_header(Range::bytes_multi(vec![(1, 100), (200, 300)]));
+/// let req = test::TestRequest::default()
+///     .insert_header(Range::Bytes(vec![
+///         ByteRangeSpec::FromTo(1, 100),
+///         ByteRangeSpec::From(200),
+///     ]))
+///     .insert_header(Range::Unregistered("letters".to_owned(), "a-f".to_owned()))
+///     .insert_header(Range::bytes(1, 100))
+///     .insert_header(Range::bytes_multi(vec![(1, 100), (200, 300)]))
+///     .to_http_request();
+/// # let _ = req;
 /// ```
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum Range {