--- a/dom/mathml/nsMathMLElement.cpp
+++ b/dom/mathml/nsMathMLElement.cpp
@@ -468,16 +468,17 @@ nsMathMLElement::ParseNumericValue(const
   else if (unit.EqualsLiteral("em")) cssUnit = eCSSUnit_EM;
   else if (unit.EqualsLiteral("ex")) cssUnit = eCSSUnit_XHeight;
   else if (unit.EqualsLiteral("px")) cssUnit = eCSSUnit_Pixel;
   else if (unit.EqualsLiteral("in")) cssUnit = eCSSUnit_Inch;
   else if (unit.EqualsLiteral("cm")) cssUnit = eCSSUnit_Centimeter;
   else if (unit.EqualsLiteral("mm")) cssUnit = eCSSUnit_Millimeter;
   else if (unit.EqualsLiteral("pt")) cssUnit = eCSSUnit_Point;
   else if (unit.EqualsLiteral("pc")) cssUnit = eCSSUnit_Pica;
+  else if (unit.EqualsLiteral("q")) cssUnit = eCSSUnit_Quarter;
   else { // unexpected unit
     if (!(aFlags & PARSE_SUPPRESS_WARNINGS)) {
       ReportLengthParseError(aString, aDocument);
     }
     return false;
   }
 
   aCSSValue.SetFloatValue(floatValue, cssUnit);

--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -7445,16 +7445,17 @@ const UnitInfo UnitData[] = {
   { STR_WITH_LEN("rem"), eCSSUnit_RootEM, VARIANT_LENGTH },
   { STR_WITH_LEN("mm"), eCSSUnit_Millimeter, VARIANT_LENGTH },
   { STR_WITH_LEN("mozmm"), eCSSUnit_PhysicalMillimeter, VARIANT_LENGTH },
   { STR_WITH_LEN("vw"), eCSSUnit_ViewportWidth, VARIANT_LENGTH },
   { STR_WITH_LEN("vh"), eCSSUnit_ViewportHeight, VARIANT_LENGTH },
   { STR_WITH_LEN("vmin"), eCSSUnit_ViewportMin, VARIANT_LENGTH },
   { STR_WITH_LEN("vmax"), eCSSUnit_ViewportMax, VARIANT_LENGTH },
   { STR_WITH_LEN("pc"), eCSSUnit_Pica, VARIANT_LENGTH },
+  { STR_WITH_LEN("q"), eCSSUnit_Quarter, VARIANT_LENGTH },
   { STR_WITH_LEN("deg"), eCSSUnit_Degree, VARIANT_ANGLE },
   { STR_WITH_LEN("grad"), eCSSUnit_Grad, VARIANT_ANGLE },
   { STR_WITH_LEN("rad"), eCSSUnit_Radian, VARIANT_ANGLE },
   { STR_WITH_LEN("turn"), eCSSUnit_Turn, VARIANT_ANGLE },
   { STR_WITH_LEN("hz"), eCSSUnit_Hertz, VARIANT_FREQUENCY },
   { STR_WITH_LEN("khz"), eCSSUnit_Kilohertz, VARIANT_FREQUENCY },
   { STR_WITH_LEN("s"), eCSSUnit_Seconds, VARIANT_TIME },
   { STR_WITH_LEN("ms"), eCSSUnit_Milliseconds, VARIANT_TIME }

--- a/layout/style/nsCSSValue.cpp
+++ b/layout/style/nsCSSValue.cpp
@@ -327,16 +327,17 @@ nscoord nsCSSValue::GetPixelLength() con
   double scaleFactor;
   switch (mUnit) {
   case eCSSUnit_Pixel: return nsPresContext::CSSPixelsToAppUnits(mValue.mFloat);
   case eCSSUnit_Pica: scaleFactor = 16.0; break;
   case eCSSUnit_Point: scaleFactor = 4/3.0; break;
   case eCSSUnit_Inch: scaleFactor = 96.0; break;
   case eCSSUnit_Millimeter: scaleFactor = 96/25.4; break;
   case eCSSUnit_Centimeter: scaleFactor = 96/2.54; break;
+  case eCSSUnit_Quarter: scaleFactor = 96/101.6; break;
   default:
     NS_ERROR("should never get here");
     return 0;
   }
   return nsPresContext::CSSPixelsToAppUnits(float(mValue.mFloat*scaleFactor));
 }
 
 void nsCSSValue::DoReset()
@@ -1775,16 +1776,17 @@ nsCSSValue::AppendToString(nsCSSProperty
     case eCSSUnit_GridTemplateAreas:     break;
 
     case eCSSUnit_Inch:         aResult.AppendLiteral("in");   break;
     case eCSSUnit_Millimeter:   aResult.AppendLiteral("mm");   break;
     case eCSSUnit_PhysicalMillimeter: aResult.AppendLiteral("mozmm");   break;
     case eCSSUnit_Centimeter:   aResult.AppendLiteral("cm");   break;
     case eCSSUnit_Point:        aResult.AppendLiteral("pt");   break;
     case eCSSUnit_Pica:         aResult.AppendLiteral("pc");   break;
+    case eCSSUnit_Quarter:      aResult.AppendLiteral("q");    break;
 
     case eCSSUnit_ViewportWidth:  aResult.AppendLiteral("vw");   break;
     case eCSSUnit_ViewportHeight: aResult.AppendLiteral("vh");   break;
     case eCSSUnit_ViewportMin:    aResult.AppendLiteral("vmin"); break;
     case eCSSUnit_ViewportMax:    aResult.AppendLiteral("vmax"); break;
 
     case eCSSUnit_EM:           aResult.AppendLiteral("em");   break;
     case eCSSUnit_XHeight:      aResult.AppendLiteral("ex");   break;
@@ -1957,16 +1959,17 @@ nsCSSValue::SizeOfExcludingThis(mozilla:
     case eCSSUnit_Char:
     case eCSSUnit_RootEM:
     case eCSSUnit_Point:
     case eCSSUnit_Inch:
     case eCSSUnit_Millimeter:
     case eCSSUnit_Centimeter:
     case eCSSUnit_Pica:
     case eCSSUnit_Pixel:
+    case eCSSUnit_Quarter:
     case eCSSUnit_Degree:
     case eCSSUnit_Grad:
     case eCSSUnit_Turn:
     case eCSSUnit_Radian:
     case eCSSUnit_Hertz:
     case eCSSUnit_Kilohertz:
     case eCSSUnit_Seconds:
     case eCSSUnit_Milliseconds:

--- a/layout/style/nsCSSValue.h
+++ b/layout/style/nsCSSValue.h
@@ -356,17 +356,18 @@ enum nsCSSUnit {
   eCSSUnit_RootEM       = 803,    // (float) == root element font size
 
   // Screen relative measure
   eCSSUnit_Point        = 900,    // (float) 4/3 of a CSS pixel
   eCSSUnit_Inch         = 901,    // (float) 96 CSS pixels
   eCSSUnit_Millimeter   = 902,    // (float) 96/25.4 CSS pixels
   eCSSUnit_Centimeter   = 903,    // (float) 96/2.54 CSS pixels
   eCSSUnit_Pica         = 904,    // (float) 12 points == 16 CSS pixls
-  eCSSUnit_Pixel        = 905,    // (float) CSS pixel unit
+  eCSSUnit_Quarter      = 905,    // (float) 96/101.6 CSS pixels
+  eCSSUnit_Pixel        = 906,    // (float) CSS pixel unit
 
   // Angular units
   eCSSUnit_Degree       = 1000,    // (float) 360 per circle
   eCSSUnit_Grad         = 1001,    // (float) 400 per circle
   eCSSUnit_Radian       = 1002,    // (float) 2*pi per circle
   eCSSUnit_Turn         = 1003,    // (float) 1 per circle
 
   // Frequency units

--- a/testing/web-platform/meta/html/semantics/embedded-content/the-img-element/sizes/parse-a-sizes-attribute.html.ini
+++ b/testing/web-platform/meta/html/semantics/embedded-content/the-img-element/sizes/parse-a-sizes-attribute.html.ini
@@ -1,13 +1,10 @@
 [parse-a-sizes-attribute.html]
   type: testharness
-  [<img srcset="/images/green-1x1.png?e17 50w, /images/green-16x16.png?e17 51w" sizes="1q"> ref sizes="1px" (standards mode)]
-    expected: FAIL
-
   [<img srcset="/images/green-1x1.png?e23 50w, /images/green-16x16.png?e23 51w" sizes="(),1px"> ref sizes="1px" (standards mode)]
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?e24 50w, /images/green-16x16.png?e24 51w" sizes="x(),1px"> ref sizes="1px" (standards mode)]
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?e25 50w, /images/green-16x16.png?e25 51w" sizes="{},1px"> ref sizes="1px" (standards mode)]
     expected: FAIL
@@ -169,19 +166,16 @@
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?f48 50w, /images/green-16x16.png?f48 51w" sizes="calc(1px"> ref sizes="100vw" (standards mode)]
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?f49 50w, /images/green-16x16.png?f49 51w" sizes="(min-width:0) calc(1px"> ref sizes="100vw" (standards mode)]
     expected: FAIL
 
-  [<img srcset="/images/green-1x1.png?e17 50w, /images/green-16x16.png?e17 51w" sizes="1q"> ref sizes="1px" (quirks mode)]
-    expected: FAIL
-
   [<img srcset="/images/green-1x1.png?e23 50w, /images/green-16x16.png?e23 51w" sizes="(),1px"> ref sizes="1px" (quirks mode)]
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?e24 50w, /images/green-16x16.png?e24 51w" sizes="x(),1px"> ref sizes="1px" (quirks mode)]
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?e25 50w, /images/green-16x16.png?e25 51w" sizes="{},1px"> ref sizes="1px" (quirks mode)]
     expected: FAIL
@@ -343,19 +337,16 @@
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?f48 50w, /images/green-16x16.png?f48 51w" sizes="calc(1px"> ref sizes="100vw" (quirks mode)]
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?f49 50w, /images/green-16x16.png?f49 51w" sizes="(min-width:0) calc(1px"> ref sizes="100vw" (quirks mode)]
     expected: FAIL
 
-  [<img srcset="/images/green-1x1.png?e17 50w, /images/green-16x16.png?e17 51w" sizes="1q"> ref sizes="1px" (display:none)]
-    expected: FAIL
-
   [<img srcset="/images/green-1x1.png?e23 50w, /images/green-16x16.png?e23 51w" sizes="(),1px"> ref sizes="1px" (display:none)]
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?e24 50w, /images/green-16x16.png?e24 51w" sizes="x(),1px"> ref sizes="1px" (display:none)]
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?e25 50w, /images/green-16x16.png?e25 51w" sizes="{},1px"> ref sizes="1px" (display:none)]
     expected: FAIL
@@ -517,19 +508,16 @@
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?f48 50w, /images/green-16x16.png?f48 51w" sizes="calc(1px"> ref sizes="100vw" (display:none)]
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?f49 50w, /images/green-16x16.png?f49 51w" sizes="(min-width:0) calc(1px"> ref sizes="100vw" (display:none)]
     expected: FAIL
 
-  [<img srcset="/images/green-1x1.png?e17 50w, /images/green-16x16.png?e17 51w" sizes="1q"> ref sizes="1px" (width:1000px)]
-    expected: FAIL
-
   [<img srcset="/images/green-1x1.png?e23 50w, /images/green-16x16.png?e23 51w" sizes="(),1px"> ref sizes="1px" (width:1000px)]
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?e24 50w, /images/green-16x16.png?e24 51w" sizes="x(),1px"> ref sizes="1px" (width:1000px)]
     expected: FAIL
 
   [<img srcset="/images/green-1x1.png?e25 50w, /images/green-16x16.png?e25 51w" sizes="{},1px"> ref sizes="1px" (width:1000px)]
     expected: FAIL