diff --git a/layout/generic/MathMLTextRunFactory.cpp b/layout/generic/MathMLTextRunFactory.cpp
index 8b512a7aa19..30853ca1149 100644
--- a/layout/generic/MathMLTextRunFactory.cpp
+++ b/layout/generic/MathMLTextRunFactory.cpp
@@ -596,7 +596,23 @@ MathMLTextRunFactory::RebuildTextRun(nsTransformedTextRun* aTextRun,
     mathVar = styleContext->StyleFont()->mMathVariant;
 
     if (singleCharMI && mathVar == NS_MATHML_MATHVARIANT_NONE) {
-      mathVar = NS_MATHML_MATHVARIANT_ITALIC;
+      // If the user has explicitly set a non-default value for fontstyle or
+      // fontweight, the italic mathvariant behaviour of <mi> is disabled
+      // This overrides the initial values specified in fontStyle, to avoid
+      // inconsistencies in which attributes allow CSS changes and which do not.
+      if (mFlags & MATH_FONT_WEIGHT_BOLD) {
+        fontStyle.weight = NS_FONT_WEIGHT_BOLD;
+        if (mFlags & MATH_FONT_STYLING_NORMAL) {
+          fontStyle.style = NS_FONT_STYLE_NORMAL;
+        } else {
+          fontStyle.style = NS_FONT_STYLE_ITALIC;
+        }
+      } else if (mFlags & MATH_FONT_STYLING_NORMAL) {
+        fontStyle.style = NS_FONT_STYLE_NORMAL;
+        fontStyle.weight = NS_FONT_WEIGHT_NORMAL;
+      } else {
+        mathVar = NS_MATHML_MATHVARIANT_ITALIC;
+      }
     }
 
     uint32_t ch = str[i];
diff --git a/layout/generic/MathMLTextRunFactory.h b/layout/generic/MathMLTextRunFactory.h
index addd242d4fd..91cc262591e 100644
--- a/layout/generic/MathMLTextRunFactory.h
+++ b/layout/generic/MathMLTextRunFactory.h
@@ -14,14 +14,22 @@
 class MathMLTextRunFactory : public nsTransformingTextRunFactory {
 public:
   MathMLTextRunFactory(nsTransformingTextRunFactory* aInnerTransformingTextRunFactory,
-                       uint8_t aSSTYScriptLevel)
+                       uint32_t aFlags, uint8_t aSSTYScriptLevel)
     : mInnerTransformingTextRunFactory(aInnerTransformingTextRunFactory),
+      mFlags(aFlags),
       mSSTYScriptLevel(aSSTYScriptLevel) {}
 
   virtual void RebuildTextRun(nsTransformedTextRun* aTextRun,
                               gfxContext* aRefContext) MOZ_OVERRIDE;
+  enum {
+    // Style effects which may override single character <mi> behaviour
+    MATH_FONT_STYLING_NORMAL   = 0x1, // fontstyle="normal" has been set.
+    MATH_FONT_WEIGHT_BOLD      = 0x2, // fontweight="bold" has been set.
+  };
+
 protected:
   nsAutoPtr<nsTransformingTextRunFactory> mInnerTransformingTextRunFactory;
+  uint32_t mFlags;
   uint8_t mSSTYScriptLevel;
 };
 
diff --git a/layout/generic/nsTextFrame.cpp b/layout/generic/nsTextFrame.cpp
index 3635111d8c2..7458d028bb1 100644
--- a/layout/generic/nsTextFrame.cpp
+++ b/layout/generic/nsTextFrame.cpp
@@ -1821,6 +1821,7 @@ BuildTextRunsScanner::BuildTextRunForFrames(void* aTextBuffer)
   bool anyTextTransformStyle = false;
   bool anyMathMLStyling = false;
   uint8_t sstyScriptLevel = 0;
+  uint32_t mathFlags = 0;
   uint32_t textFlags = nsTextFrameUtils::TEXT_NO_BREAKS;
 
   if (mCurrentRunContextInfo & nsTextFrameUtils::INCOMING_WHITESPACE) {
@@ -1895,13 +1896,33 @@ BuildTextRunsScanner::BuildTextRunForFrames(void* aTextBuffer)
       textFlags |= gfxTextRunFactory::TEXT_ENABLE_SPACING;
     }
     fontStyle = f->StyleFont();
+    nsIFrame* parent = mLineContainer->GetParent();
     if (NS_MATHML_MATHVARIANT_NONE != fontStyle->mMathVariant) {
       anyMathMLStyling = true;
     } else if (mLineContainer->GetStateBits() & NS_FRAME_IS_IN_SINGLE_CHAR_MI) {
       textFlags |= nsTextFrameUtils::TEXT_IS_SINGLE_CHAR_MI;
       anyMathMLStyling = true;
+      // Test for fontstyle attribute as StyleFont() may not be accurate
+      // To be consistent in terms of ignoring CSS style changes, fontweight
+      // gets checked too.
+      if (parent) {
+        nsIContent* content = parent->GetContent();
+        if (content) {
+          if (content->AttrValueIs(kNameSpaceID_None,
+                                  nsGkAtoms::fontstyle_,
+                                  NS_LITERAL_STRING("normal"),
+                                  eCaseMatters)) {
+            mathFlags |= MathMLTextRunFactory::MATH_FONT_STYLING_NORMAL;
+          }
+          if (content->AttrValueIs(kNameSpaceID_None,
+                                   nsGkAtoms::fontweight_,
+                                   NS_LITERAL_STRING("bold"),
+                                   eCaseMatters)) {
+            mathFlags |= MathMLTextRunFactory::MATH_FONT_WEIGHT_BOLD;
+          }
+        }
+      }
     }
-    nsIFrame* parent = mLineContainer->GetParent();
     if (mLineContainer->HasAnyStateBits(TEXT_IS_IN_TOKEN_MATHML)) {
       // All MathML tokens except <mtext> use 'math' script.
       if (!(parent && parent->GetContent() &&
@@ -2065,7 +2086,8 @@ BuildTextRunsScanner::BuildTextRunForFrames(void* aTextBuffer)
   }
   if (anyMathMLStyling) {
     transformingFactory =
-      new MathMLTextRunFactory(transformingFactory.forget(), sstyScriptLevel);
+      new MathMLTextRunFactory(transformingFactory.forget(), mathFlags,
+                               sstyScriptLevel);
   }
   nsTArray<nsStyleContext*> styles;
   if (transformingFactory) {
diff --git a/layout/reftests/mathml/mathvariant-3-ref.html b/layout/reftests/mathml/mathvariant-3-ref.html
index ed7d05d55e4..7a7de4b82ad 100644
--- a/layout/reftests/mathml/mathvariant-3-ref.html
+++ b/layout/reftests/mathml/mathvariant-3-ref.html
@@ -1,7 +1,7 @@
 <!doctype html>
 <html>
   <head>
-    <title>Test math-variant overrides fontstyle and fontweight</title>
+    <title>Test math-variant interaction with fontstyle and fontweight</title>
   </head>
   <body>
     <math>
@@ -10,5 +10,35 @@
         <mtext>A</mtext>
       </mrow>
     </math>
+    <p>
+    <math>
+    <mn>A</mn>
+    </math>
+    </p>
+    <p>
+    <math>
+    <mn fontweight="bold">A</mn>
+    </math>
+    </p>
+    <p>
+    <math>
+    <mn fontweight="bold" fontstyle="italic">A</mn>
+    </math>
+    </p>
+    <p>
+    <math>
+    <mn fontweight="bold" fontstyle="italic">A</mn>
+    </math>
+    </p>
+    <p>
+    <math>
+      <mn fontweight="bold" fontstyle="italic">A</mn>
+    </math>
+    </p>
+    <p>
+    <math>
+      <mn>A</mn>
+    </math>
+    </p>
   </body>
  </html>
diff --git a/layout/reftests/mathml/mathvariant-3.html b/layout/reftests/mathml/mathvariant-3.html
index 7139c7df03b..b7c7a872561 100644
--- a/layout/reftests/mathml/mathvariant-3.html
+++ b/layout/reftests/mathml/mathvariant-3.html
@@ -1,7 +1,7 @@
 <!doctype html>
 <html>
   <head>
-    <title>Test math-variant overrides fontstyle and fontweight</title>
+    <title>Test math-variant interaction with fontstyle and fontweight</title>
   </head>
   <body>
     <math>
@@ -10,6 +10,36 @@
         <mtext mathvariant="normal" fontstyle="italic">A</mtext>
       </mrow>
     </math>
+    <p>
+    <math>
+      <mi fontstyle="normal">A</mi>
+    </math>
+    </p>
+    <p>
+    <math>
+      <mi fontstyle="normal" fontweight="bold">A</mi>
+    </math>
+    </p>
+    <p>
+    <math>
+      <mi fontweight="bold">A</mi>
+    </math>
+    </p>
+    <p>
+    <math>
+      <mi fontweight="bold" fontstyle="italic">A</mi>
+    </math>
+    </p>
+    <p>
+    <math>
+      <mi fontweight="bold" style="font-style:normal">A</mi>
+    </math>
+    </p>
+    <p>
+    <math>
+      <mi fontstyle="normal" style="font-weight:bold">A</mi>
+    </math>
+    </p>
   </body>
  </html>