diff --git a/browser/base/content/test/static/browser_misused_characters_in_strings.js b/browser/base/content/test/static/browser_misused_characters_in_strings.js
index b8da986d2f63..4191cc966e4c 100644
--- a/browser/base/content/test/static/browser_misused_characters_in_strings.js
+++ b/browser/base/content/test/static/browser_misused_characters_in_strings.js
@@ -57,6 +57,17 @@ let gExceptionsList = [
     key: "ImportMapExternalNotSupported",
     type: "single-quote",
   },
+  {
+    file: "dom.properties",
+    key: "MathML_DeprecatedMathVariantWarning",
+    type: "single-quote",
+  },
+  // dom.properties is packaged twice so we need to have two exceptions for this string.
+  {
+    file: "dom.properties",
+    key: "MathML_DeprecatedMathVariantWarning",
+    type: "single-quote",
+  },
 ];
 
 /**
diff --git a/dom/base/nsDeprecatedOperationList.h b/dom/base/nsDeprecatedOperationList.h
index 61b587e90c24..56178fa7a131 100644
--- a/dom/base/nsDeprecatedOperationList.h
+++ b/dom/base/nsDeprecatedOperationList.h
@@ -48,6 +48,7 @@ DEPRECATED_OPERATION(MozInputSource)
 DEPRECATED_OPERATION(InitMouseEvent)
 DEPRECATED_OPERATION(InitNSMouseEvent)
 DEPRECATED_OPERATION(MathML_DeprecatedMathSpaceValue)
+DEPRECATED_OPERATION(MathML_DeprecatedMathVariant)
 DEPRECATED_OPERATION(MathML_DeprecatedStixgeneralOperatorStretching)
 DEPRECATED_OPERATION(FormSubmissionUntrustedEvent)
 DEPRECATED_OPERATION(ElementSetCapture)
diff --git a/dom/locales/en-US/chrome/dom/dom.properties b/dom/locales/en-US/chrome/dom/dom.properties
index bfb4a2160980..5b6114b22b7b 100644
--- a/dom/locales/en-US/chrome/dom/dom.properties
+++ b/dom/locales/en-US/chrome/dom/dom.properties
@@ -389,6 +389,8 @@ InitNSMouseEventWarning=initNSMouseEvent() is deprecated. Use the MouseEvent() c
 # LOCALIZATION NOTE: Do not translate veryverythinmathspace, verythinmathspace,
 # thinmathspace, mediummathspace, thickmathspace, verythickmathspace, veryverythickmathspace and MathML.
 MathML_DeprecatedMathSpaceValueWarning=“veryverythinmathspace”, “verythinmathspace”, “thinmathspace”, “mediummathspace”, “thickmathspace”, “verythickmathspace” and “veryverythickmathspace” are deprecated values for MathML lengths and will be removed at a future date.
+# LOCALIZATION NOTE: Do not translate mathvariant or MathML. %S is the deprecated value of the mathvariant attribute.
+MathML_DeprecatedMathVariantWarning=“mathvariant='%S'” on MathML elements is deprecated and will be removed at a future date.
 # LOCALIZATION NOTE: Do not translate MathML and STIXGeneral. %S is a documentation URL.
 MathML_DeprecatedStixgeneralOperatorStretchingWarning=Support for rendering stretched MathML operators with STIXGeneral fonts is deprecated and may be removed at a future date. For details about newer fonts that will continue to be supported, see %S
 FormSubmissionUntrustedEventWarning=Form submission via untrusted submit event is deprecated and will be removed at a future date.
diff --git a/dom/mathml/MathMLElement.cpp b/dom/mathml/MathMLElement.cpp
index a46ee33ece5f..08b7571f26a1 100644
--- a/dom/mathml/MathMLElement.cpp
+++ b/dom/mathml/MathMLElement.cpp
@@ -519,6 +519,20 @@ void MathMLElement::MapGlobalMathMLAttributesInto(
           StyleMathVariant::Stretched};
       for (uint32_t i = 0; i < ArrayLength(sizes); ++i) {
         if (str.LowerCaseEqualsASCII(sizes[i])) {
+          if (values[i] != StyleMathVariant::Normal) {
+            // Warn about deprecated mathvariant attribute values. Strictly
+            // speaking, we should also warn about mathvariant="normal" if the
+            // element is not an <mi>. However this would require exposing the
+            // tag name via aBuilder. Moreover, this use case is actually to
+            // revert the effect of a non-normal mathvariant value on an
+            // ancestor element, which should consequently have already
+            // triggered a warning.
+            AutoTArray<nsString, 1> params;
+            params.AppendElement(str);
+            aBuilder.Document().WarnOnceAbout(
+                dom::DeprecatedOperations::eMathML_DeprecatedMathVariant, false,
+                params);
+          }
           aBuilder.SetKeywordValue(eCSSProperty__moz_math_variant, values[i]);
           break;
         }
diff --git a/layout/mathml/tests/mochitest.ini b/layout/mathml/tests/mochitest.ini
index ad0b78f08f16..6ac097c46eed 100644
--- a/layout/mathml/tests/mochitest.ini
+++ b/layout/mathml/tests/mochitest.ini
@@ -2,6 +2,7 @@
 support-files =
   file_disabled_iframe.html
 
+[test_bug1845461.html]
 [test_bug330964.html]
 [test_bug553917.html]
 [test_bug827713-2.html]
diff --git a/layout/mathml/tests/test_bug1845461.html b/layout/mathml/tests/test_bug1845461.html
new file mode 100644
index 000000000000..26076602c1e0
--- /dev/null
+++ b/layout/mathml/tests/test_bug1845461.html
@@ -0,0 +1,36 @@
+<!DOCTYPE HTML>
+<!--
+    https://bugzilla.mozilla.org/show_bug.cgi?id=1845461
+  -->
+<html>
+  <head>
+    <title>Test for Bug 1845461</title>
+    <script src="/tests/SimpleTest/SimpleTest.js"></script>
+    <script src="/tests/SimpleTest/EventUtils.js"></script>
+    <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+    <script type="application/javascript">
+      const allow_legacy_mathvariant = !SpecialPowers.getBoolPref('mathml.legacy_mathvariant_attribute.disabled');
+      let errorMessages = [];
+      SpecialPowers.registerConsoleListener(
+          function (msg) {
+              if (msg.message == "SENTINEL") {
+                  is(errorMessages.length, allow_legacy_mathvariant ? 1 : 0,
+                     "number of errors");
+                  if (allow_legacy_mathvariant) {
+                      is(errorMessages[0], "“mathvariant='fraktur'” on MathML elements is deprecated and will be removed at a future date.",
+                        "error message");
+                  }
+                  SimpleTest.finish();
+              } else if (msg.isScriptError) {
+                  errorMessages.push(msg.errorMessage);
+              }
+          });
+      SimpleTest.waitForExplicitFinish();
+    </script>
+  </head>
+  <body onload="SpecialPowers.postConsoleSentinel();">
+    <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1845461">Mozilla Bug 1845461</a>
+    <math><mi mathvariant="normal">A</mi></math>
+    <math><mi mathvariant="fraktur">A</mi></math>
+  </body>
+</html>