--- a/chrome/browser/ungoogled_flag_entries.h
+++ b/chrome/browser/ungoogled_flag_entries.h
@@ -92,4 +92,8 @@
      "Disable GREASE for TLS",
      "Turn off GREASE (Generate Random Extensions And Sustain Extensibility) for TLS connections. ungoogled-chromium flag.",
      kOsAll, SINGLE_VALUE_TYPE("disable-grease-tls")},
+    {"http-accept-header",
+     "Custom HTTP Accept Header",
+     "Set a custom value for the Accept header which is sent by the browser with every HTTP request.  (e.g. `text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8`). ungoogled-chromium flag.",
+     kOsAll, ORIGIN_LIST_VALUE_TYPE("http-accept-header", "")},
 #endif  // CHROME_BROWSER_UNGOOGLED_FLAG_ENTRIES_H_
--- a/components/flags_ui/flags_state.cc
+++ b/components/flags_ui/flags_state.cc
@@ -223,6 +223,7 @@ std::string GetCombinedOriginListValue(c
   const std::string new_value =
       flags_storage.GetOriginListFlag(internal_entry_name);
   if (command_line_switch == "custom-ntp") return existing_value.empty() ? new_value : existing_value;
+  if (command_line_switch == "http-accept-header") return existing_value.empty() ? new_value : existing_value;
   return CombineAndSanitizeOriginLists(existing_value, new_value);
 }
 
@@ -462,6 +463,7 @@ void FlagsState::SetOriginListFlag(const
                                    FlagsStorage* flags_storage) {
   const std::string new_value =
       internal_name == "custom-ntp" ? value :
+      internal_name == "http-accept-header" ? value :
       CombineAndSanitizeOriginLists(std::string(), value);
   flags_storage->SetOriginListFlag(internal_name, new_value);
 
--- a/content/public/browser/frame_accept_header.cc
+++ b/content/public/browser/frame_accept_header.cc
@@ -4,6 +4,7 @@
 
 #include "content/public/browser/frame_accept_header.h"
 
+#include "base/command_line.h"
 #include "content/browser/web_package/signed_exchange_consts.h"
 #include "content/browser/web_package/signed_exchange_utils.h"
 #include "content/common/content_constants_internal.h"
@@ -12,6 +13,9 @@ namespace content {
 
 std::string FrameAcceptHeaderValue(bool allow_sxg_responses,
                                    BrowserContext* browser_context) {
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch("http-accept-header"))
+    return base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII("http-accept-header");
+
   std::string header_value = kFrameAcceptHeaderValue;
 
   if (allow_sxg_responses &&
