diff --git a/dom/base/nsDeprecatedOperationList.h b/dom/base/nsDeprecatedOperationList.h
index 2a0ff32d7dcd..e3ca65f1bbcf 100644
--- a/dom/base/nsDeprecatedOperationList.h
+++ b/dom/base/nsDeprecatedOperationList.h
@@ -49,3 +49,6 @@ DEPRECATED_OPERATION(MozfullscreenchangeDeprecatedPrefix)
 DEPRECATED_OPERATION(MozfullscreenerrorDeprecatedPrefix)
 DEPRECATED_OPERATION(External_AddSearchProvider)
 DEPRECATED_OPERATION(MouseEvent_MozPressure)
+DEPRECATED_OPERATION(MathML_DeprecatedLineThicknessValue)
+DEPRECATED_OPERATION(MathML_DeprecatedMathSizeValue)
+DEPRECATED_OPERATION(MathML_DeprecatedMathSpaceValue)
diff --git a/dom/locales/en-US/chrome/dom/dom.properties b/dom/locales/en-US/chrome/dom/dom.properties
index c8d083aa6b09..2e95363ff964 100644
--- a/dom/locales/en-US/chrome/dom/dom.properties
+++ b/dom/locales/en-US/chrome/dom/dom.properties
@@ -387,3 +387,10 @@ MozfullscreenerrorDeprecatedPrefixWarning=onmozfullscreenerror is deprecated.
 External_AddSearchProviderWarning=AddSearchProvider is deprecated.
 # LOCALIZATION NOTE: Do not translate "MouseEvent.mozPressure" and "PointerEvent.pressure".
 MouseEvent_MozPressureWarning=MouseEvent.mozPressure is deprecated. Use PointerEvent.pressure instead.
+# LOCALIZATION NOTE: Do not translate thin, medium, thick and linethickness.
+MathML_DeprecatedLineThicknessValueWarning=“thin”, “medium” and “thick” are deprecated values for the linethickness attribute and will be removed at a future date.
+# LOCALIZATION NOTE: Do not translate small, normal, big and mathsize.
+MathML_DeprecatedMathSizeValueWarning=“small”, “normal” and “big” are deprecated values for the mathsize attribute and will be removed at a future date.
+# 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.
diff --git a/dom/mathml/nsMathMLElement.cpp b/dom/mathml/nsMathMLElement.cpp
index b31dd1e89bd6..79988b756a76 100644
--- a/dom/mathml/nsMathMLElement.cpp
+++ b/dom/mathml/nsMathMLElement.cpp
@@ -187,7 +187,8 @@ nsMapRuleToAttributesFunc nsMathMLElement::GetAttributeMappingFunction() const {
 /* static */
 bool nsMathMLElement::ParseNamedSpaceValue(const nsString& aString,
                                            nsCSSValue& aCSSValue,
-                                           uint32_t aFlags) {
+                                           uint32_t aFlags,
+                                           const Document& aDocument) {
   if (StaticPrefs::mathml_mathspace_names_disabled()) {
     return false;
   }
@@ -225,6 +226,7 @@ bool nsMathMLElement::ParseNamedSpaceValue(const nsString& aString,
     }
   }
   if (0 != i) {
+    aDocument.WarnOnceAbout(dom::Document::eMathML_DeprecatedMathSpaceValue);
     aCSSValue.SetFloatValue(float(i) / float(18), eCSSUnit_EM);
     return true;
   }
@@ -285,7 +287,7 @@ bool nsMathMLElement::ParseNumericValue(const nsString& aString,
     return false;
   }
 
-  if (ParseNamedSpaceValue(str, aCSSValue, aFlags)) {
+  if (aDocument && ParseNamedSpaceValue(str, aCSSValue, aFlags, *aDocument)) {
     return true;
   }
 
@@ -546,6 +548,8 @@ void nsMathMLElement::MapMathMLAttributesInto(
       str.CompressWhitespace();
       for (uint32_t i = 0; i < ArrayLength(sizes); ++i) {
         if (str.EqualsASCII(sizes[i])) {
+          aDecls.Document()->WarnOnceAbout(
+              dom::Document::eMathML_DeprecatedMathSizeValue);
           aDecls.SetKeywordValue(eCSSProperty_font_size, values[i]);
           break;
         }
diff --git a/dom/mathml/nsMathMLElement.h b/dom/mathml/nsMathMLElement.h
index c4eb0ecfee0c..30b15c119a81 100644
--- a/dom/mathml/nsMathMLElement.h
+++ b/dom/mathml/nsMathMLElement.h
@@ -54,7 +54,8 @@ class nsMathMLElement final : public nsMathMLElementBase,
     CONVERT_UNITLESS_TO_PERCENT = 0x08
   };
   static bool ParseNamedSpaceValue(const nsString& aString,
-                                   nsCSSValue& aCSSValue, uint32_t aFlags);
+                                   nsCSSValue& aCSSValue, uint32_t aFlags,
+                                   const Document& aDocument);
 
   static bool ParseNumericValue(const nsString& aString, nsCSSValue& aCSSValue,
                                 uint32_t aFlags, Document* aDocument);
diff --git a/layout/mathml/nsMathMLmfracFrame.cpp b/layout/mathml/nsMathMLmfracFrame.cpp
index c3a65d171c9f..fb8a0bf8fce4 100644
--- a/layout/mathml/nsMathMLmfracFrame.cpp
+++ b/layout/mathml/nsMathMLmfracFrame.cpp
@@ -111,6 +111,7 @@ nscoord nsMathMLmfracFrame::CalcLineThickness(nsPresContext* aPresContext,
                         nsMathMLElement::PARSE_ALLOW_UNITLESS, aPresContext,
                         aComputedStyle, aFontSizeInflation);
     } else {
+      bool isDeprecatedLineThicknessValue = true;
       if (aThicknessAttribute.EqualsLiteral("thin")) {
         lineThickness = NSToCoordFloor(defaultThickness * THIN_FRACTION_LINE);
         minimumThickness = onePixel * THIN_FRACTION_LINE_MINIMUM_PIXELS;
@@ -131,11 +132,16 @@ nscoord nsMathMLmfracFrame::CalcLineThickness(nsPresContext* aPresContext,
         }
       } else {
         // length value
+        isDeprecatedLineThicknessValue = false;
         lineThickness = defaultThickness;
         ParseNumericValue(aThicknessAttribute, &lineThickness,
                           nsMathMLElement::PARSE_ALLOW_UNITLESS, aPresContext,
                           aComputedStyle, aFontSizeInflation);
       }
+      if (isDeprecatedLineThicknessValue) {
+        mContent->OwnerDoc()->WarnOnceAbout(
+            dom::Document::eMathML_DeprecatedLineThicknessValue);
+      }
     }
   }
 
diff --git a/layout/mathml/nsMathMLmfracFrame.h b/layout/mathml/nsMathMLmfracFrame.h
index d62e42b8d229..8788a3395634 100644
--- a/layout/mathml/nsMathMLmfracFrame.h
+++ b/layout/mathml/nsMathMLmfracFrame.h
@@ -82,12 +82,11 @@ class nsMathMLmfracFrame final : public nsMathMLContainerFrame {
   virtual nscoord FixInterFrameSpacing(ReflowOutput& aDesiredSize) override;
 
   // helper to translate the thickness attribute into a usable form
-  static nscoord CalcLineThickness(nsPresContext* aPresContext,
-                                   ComputedStyle* aComputedStyle,
-                                   nsString& aThicknessAttribute,
-                                   nscoord onePixel,
-                                   nscoord aDefaultRuleThickness,
-                                   float aFontSizeInflation);
+  nscoord CalcLineThickness(nsPresContext* aPresContext,
+                            ComputedStyle* aComputedStyle,
+                            nsString& aThicknessAttribute, nscoord onePixel,
+                            nscoord aDefaultRuleThickness,
+                            float aFontSizeInflation);
 
   uint8_t ScriptIncrement(nsIFrame* aFrame) override;
 
diff --git a/layout/mathml/nsMathMLmpaddedFrame.cpp b/layout/mathml/nsMathMLmpaddedFrame.cpp
index 4128a53fdf4b..7c823633c471 100644
--- a/layout/mathml/nsMathMLmpaddedFrame.cpp
+++ b/layout/mathml/nsMathMLmpaddedFrame.cpp
@@ -208,7 +208,8 @@ bool nsMathMLmpaddedFrame::ParseAttribute(nsString& aString, int32_t& aSign,
 
     // see if the unit is a named-space
     if (nsMathMLElement::ParseNamedSpaceValue(
-            unit, aCSSValue, nsMathMLElement::PARSE_ALLOW_NEGATIVE)) {
+            unit, aCSSValue, nsMathMLElement::PARSE_ALLOW_NEGATIVE,
+            *mContent->OwnerDoc())) {
       // re-scale properly, and we know that the unit of the named-space is 'em'
       floatValue *= aCSSValue.GetFloatValue();
       aCSSValue.SetFloatValue(floatValue, eCSSUnit_EM);
diff --git a/layout/mathml/nsMathMLmpaddedFrame.h b/layout/mathml/nsMathMLmpaddedFrame.h
index e14edca2d320..b1097719fd8f 100644
--- a/layout/mathml/nsMathMLmpaddedFrame.h
+++ b/layout/mathml/nsMathMLmpaddedFrame.h
@@ -86,8 +86,8 @@ class nsMathMLmpaddedFrame final : public nsMathMLContainerFrame {
   // helpers to process the attributes
   void ProcessAttributes();
 
-  static bool ParseAttribute(nsString& aString, int32_t& aSign,
-                             nsCSSValue& aCSSValue, int32_t& aPseudoUnit);
+  bool ParseAttribute(nsString& aString, int32_t& aSign, nsCSSValue& aCSSValue,
+                      int32_t& aPseudoUnit);
 
   void UpdateValue(int32_t aSign, int32_t aPseudoUnit,
                    const nsCSSValue& aCSSValue,