diff --git a/layout/inspector/inDOMUtils.cpp b/layout/inspector/inDOMUtils.cpp
index 58afc5f76..800201ce2 100644
--- a/layout/inspector/inDOMUtils.cpp
+++ b/layout/inspector/inDOMUtils.cpp
@@ -905,7 +905,7 @@ inDOMUtils::CssPropertySupportsType(const nsAString& aProperty, uint32_t aType,
     break;
   case TYPE_NUMBER:
     // Include integers under "number"?
-    variant = VARIANT_NUMBER | VARIANT_INTEGER;
+    variant = VARIANT_NUMBER | VARIANT_INTEGER | VARIANT_OPACITY;
     break;
   default:
     // Unknown type
diff --git a/layout/style/nsCSSParser.cpp b/layout/style/nsCSSParser.cpp
index cdc84d34c..0b906f757 100644
--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -7789,6 +7789,7 @@ CSSParserImpl::ParseNonNegativeVariant(nsCSSValue& aValue,
                                VARIANT_NUMBER |
                                VARIANT_LENGTH |
                                VARIANT_PERCENT |
+                               VARIANT_OPACITY |
                                VARIANT_INTEGER)) == 0,
              "need to update code below to handle additional variants");
 
@@ -7829,6 +7830,7 @@ CSSParserImpl::ParseOneOrLargerVariant(nsCSSValue& aValue,
   // that we specifically handle.
   MOZ_ASSERT((aVariantMask & ~(VARIANT_ALL_NONNUMERIC |
                                VARIANT_NUMBER |
+                               VARIANT_OPACITY |
                                VARIANT_INTEGER)) == 0,
              "need to update code below to handle additional variants");
 
@@ -7957,9 +7959,9 @@ CSSParserImpl::ParseVariant(nsCSSValue& aValue,
       }
     }
   }
-  // Check VARIANT_NUMBER and VARIANT_INTEGER before VARIANT_LENGTH or
-  // VARIANT_ZERO_ANGLE.
-  if (((aVariantMask & VARIANT_NUMBER) != 0) &&
+  // Check VARIANT_NUMBER, number tokens for VARIANT_OPACITY, and 
+  // VARIANT_INTEGER before VARIANT_LENGTH or VARIANT_ZERO_ANGLE.
+  if (((aVariantMask & (VARIANT_NUMBER | VARIANT_OPACITY)) != 0) &&
       (eCSSToken_Number == tk->mType)) {
     aValue.SetFloatValue(tk->mNumber, eCSSUnit_Number);
     return CSSParseResult::Ok;
@@ -7969,6 +7971,7 @@ CSSParserImpl::ParseVariant(nsCSSValue& aValue,
     aValue.SetIntValue(tk->mInteger, eCSSUnit_Integer);
     return CSSParseResult::Ok;
   }
+
   if (((aVariantMask & (VARIANT_LENGTH | VARIANT_ANGLE |
                         VARIANT_FREQUENCY | VARIANT_TIME)) != 0 &&
        eCSSToken_Dimension == tk->mType) ||
@@ -7994,6 +7997,15 @@ CSSParserImpl::ParseVariant(nsCSSValue& aValue,
     aValue.SetPercentValue(tk->mNumber);
     return CSSParseResult::Ok;
   }
+  // We need to store eCSSToken_Percentage in eCSSUnit_Number in order to 
+  // serialize opacity according to spec. All percentage tokens are stored 
+  // as floats, so no type conversion is needed to make this possible. 
+  // Percentage tokens have to be evaluated later than number tokens.
+  if (((aVariantMask & VARIANT_OPACITY) != 0) &&
+      (eCSSToken_Percentage == tk->mType)) {
+    aValue.SetFloatValue(tk->mNumber, eCSSUnit_Number);
+    return CSSParseResult::Ok;
+  }
   if (mUnitlessLengthQuirk) { // NONSTANDARD: Nav interprets unitless numbers as px
     if (((aVariantMask & VARIANT_LENGTH) != 0) &&
         (eCSSToken_Number == tk->mType)) {
diff --git a/layout/style/nsCSSPropList.h b/layout/style/nsCSSPropList.h
index f62aa3827..890019245 100644
--- a/layout/style/nsCSSPropList.h
+++ b/layout/style/nsCSSPropList.h
@@ -1703,7 +1703,7 @@ CSS_PROP_SVG(
     FillOpacity,
     CSS_PROPERTY_PARSE_VALUE,
     "",
-    VARIANT_HN | VARIANT_OPENTYPE_SVG_KEYWORD,
+    VARIANT_INHERIT | VARIANT_OPACITY | VARIANT_OPENTYPE_SVG_KEYWORD,
     kContextOpacityKTable,
     offsetof(nsStyleSVG, mFillOpacity),
     eStyleAnimType_float)
@@ -1841,7 +1841,7 @@ CSS_PROP_SVGRESET(
     FloodOpacity,
     CSS_PROPERTY_PARSE_VALUE,
     "",
-    VARIANT_HN,
+    VARIANT_INHERIT | VARIANT_OPACITY,
     nullptr,
     offsetof(nsStyleSVGReset, mFloodOpacity),
     eStyleAnimType_float)
@@ -3070,7 +3070,7 @@ CSS_PROP_EFFECTS(
         CSS_PROPERTY_CAN_ANIMATE_ON_COMPOSITOR |
         CSS_PROPERTY_CREATES_STACKING_CONTEXT,
     "",
-    VARIANT_HN,
+    VARIANT_INHERIT | VARIANT_OPACITY,
     nullptr,
     offsetof(nsStyleEffects, mOpacity),
     eStyleAnimType_float)
@@ -3761,7 +3761,7 @@ CSS_PROP_SVGRESET(
     StopOpacity,
     CSS_PROPERTY_PARSE_VALUE,
     "",
-    VARIANT_HN,
+    VARIANT_INHERIT | VARIANT_OPACITY,
     nullptr,
     offsetof(nsStyleSVGReset, mStopOpacity),
     eStyleAnimType_float)
@@ -3836,7 +3836,7 @@ CSS_PROP_SVG(
     StrokeOpacity,
     CSS_PROPERTY_PARSE_VALUE,
     "",
-    VARIANT_HN | VARIANT_OPENTYPE_SVG_KEYWORD,
+    VARIANT_INHERIT | VARIANT_OPACITY | VARIANT_OPENTYPE_SVG_KEYWORD,
     kContextOpacityKTable,
     offsetof(nsStyleSVG, mStrokeOpacity),
     eStyleAnimType_float)
diff --git a/layout/style/nsCSSProps.h b/layout/style/nsCSSProps.h
index aabbac07a..34d457c08 100644
--- a/layout/style/nsCSSProps.h
+++ b/layout/style/nsCSSProps.h
@@ -43,6 +43,7 @@
 #define VARIANT_IDENTIFIER      0x002000  // D
 #define VARIANT_IDENTIFIER_NO_INHERIT 0x004000 // like above, but excluding
 // 'inherit' and 'initial'
+#define VARIANT_OPACITY         0x008000  // Take floats and percents as input, output float.
 #define VARIANT_AUTO            0x010000  // A
 #define VARIANT_INHERIT         0x020000  // H eCSSUnit_Initial, eCSSUnit_Inherit, eCSSUnit_Unset
 #define VARIANT_NONE            0x040000  // O
diff --git a/layout/style/test/property_database.js b/layout/style/test/property_database.js
index 2d6352148..bc4383630 100644
--- a/layout/style/test/property_database.js
+++ b/layout/style/test/property_database.js
@@ -3540,7 +3540,7 @@ var gCSSProperties = {
     inherited: false,
     type: CSS_TYPE_LONGHAND,
     initial_values: [ "1", "17", "397.376", "3e1", "3e+1", "3e0", "3e+0", "3e-0" ],
-    other_values: [ "0", "0.4", "0.0000", "-3", "3e-1" ],
+    other_values: [ "0", "0.4", "0.0000", "-3", "3e-1" "-100%", "50%" ],
     invalid_values: [ "0px", "1px" ]
   },
   "-moz-orient": {
@@ -4272,8 +4272,8 @@ var gCSSProperties = {
     domProp: "fillOpacity",
     inherited: true,
     type: CSS_TYPE_LONGHAND,
-    initial_values: [ "1", "2.8", "1.000", "context-fill-opacity", "context-stroke-opacity" ],
-    other_values: [ "0", "0.3", "-7.3" ],
+    initial_values: [ "1", "2.8", "1.000", "300%", "context-fill-opacity", "context-stroke-opacity" ],
+    other_values: [ "0", "0.3", "-7.3", "-100%", "50%" ],
     invalid_values: []
   },
   "fill-rule": {
@@ -4305,8 +4305,8 @@ var gCSSProperties = {
     domProp: "floodOpacity",
     inherited: false,
     type: CSS_TYPE_LONGHAND,
-    initial_values: [ "1", "2.8", "1.000" ],
-    other_values: [ "0", "0.3", "-7.3" ],
+    initial_values: [ "1", "2.8", "1.000", "300%" ],
+    other_values: [ "0", "0.3", "-7.3", "-100%", "50%" ],
     invalid_values: []
   },
   "image-rendering": {
@@ -4380,8 +4380,8 @@ var gCSSProperties = {
     domProp: "stopOpacity",
     inherited: false,
     type: CSS_TYPE_LONGHAND,
-    initial_values: [ "1", "2.8", "1.000" ],
-    other_values: [ "0", "0.3", "-7.3" ],
+    initial_values: [ "1", "2.8", "1.000", "300%" ],
+    other_values: [ "0", "0.3", "-7.3", "-100%", "50%" ],
     invalid_values: []
   },
   "stroke": {
@@ -4436,8 +4436,8 @@ var gCSSProperties = {
     domProp: "strokeOpacity",
     inherited: true,
     type: CSS_TYPE_LONGHAND,
-    initial_values: [ "1", "2.8", "1.000", "context-fill-opacity", "context-stroke-opacity" ],
-    other_values: [ "0", "0.3", "-7.3" ],
+    initial_values: [ "1", "2.8", "1.000", "300%", "context-fill-opacity", "context-stroke-opacity" ],
+    other_values: [ "0", "0.3", "-7.3", "-100%", "50% ],
     invalid_values: []
   },
   "stroke-width": {