--- a/gfx/src/nsColor.cpp
+++ b/gfx/src/nsColor.cpp
@@ -1,15 +1,16 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/ArrayUtils.h"         // for ArrayLength
 #include "mozilla/mozalloc.h"           // for operator delete, etc
+#include "mozilla/MathAlgorithms.h"
 
 #include "nsColor.h"
 #include <sys/types.h>                  // for int32_t
 #include "nsColorNames.h"               // for nsColorNames
 #include "nsDebug.h"                    // for NS_ASSERTION, etc
 #include "nsStaticNameTable.h"
 #include "nsString.h"                   // for nsAutoCString, nsString, etc
 #include "nscore.h"                     // for nsAString, etc
@@ -251,16 +252,69 @@ NS_ComposeColors(nscolor aBG, nscolor aF
   }
   MOZ_BLEND(r, NS_GET_R(aBG), NS_GET_R(aFG), blendAlpha);
   MOZ_BLEND(g, NS_GET_G(aBG), NS_GET_G(aFG), blendAlpha);
   MOZ_BLEND(b, NS_GET_B(aBG), NS_GET_B(aFG), blendAlpha);
 
   return NS_RGBA(r, g, b, a);
 }
 
+namespace mozilla {
+
+static uint32_t
+BlendColorComponent(uint32_t aBg, uint32_t aFg, uint32_t aFgAlpha)
+{
+  return RoundingDivideBy255(aBg * (255 - aFgAlpha) + aFg * aFgAlpha);
+}
+
+nscolor
+LinearBlendColors(nscolor aBg, nscolor aFg, uint_fast8_t aFgRatio)
+{
+  // Common case that either pure background or pure foreground
+  if (aFgRatio == 0) {
+    return aBg;
+  }
+  if (aFgRatio == 255) {
+    return aFg;
+  }
+  // Common case that alpha channel is equal (usually both are opaque)
+  if (NS_GET_A(aBg) == NS_GET_A(aFg)) {
+    auto r = BlendColorComponent(NS_GET_R(aBg), NS_GET_R(aFg), aFgRatio);
+    auto g = BlendColorComponent(NS_GET_G(aBg), NS_GET_G(aFg), aFgRatio);
+    auto b = BlendColorComponent(NS_GET_B(aBg), NS_GET_B(aFg), aFgRatio);
+    return NS_RGBA(r, g, b, NS_GET_A(aFg));
+  }
+
+  constexpr float kFactor = 1.0f / 255.0f;
+
+  float p1 = kFactor * (255 - aFgRatio);
+  float a1 = kFactor * NS_GET_A(aBg);
+  float r1 = a1 * NS_GET_R(aBg);
+  float g1 = a1 * NS_GET_G(aBg);
+  float b1 = a1 * NS_GET_B(aBg);
+
+  float p2 = 1.0f - p1;
+  float a2 = kFactor * NS_GET_A(aFg);
+  float r2 = a2 * NS_GET_R(aFg);
+  float g2 = a2 * NS_GET_G(aFg);
+  float b2 = a2 * NS_GET_B(aFg);
+
+  float a = p1 * a1 + p2 * a2;
+  if (a == 0.0) {
+    return NS_RGBA(0, 0, 0, 0);
+  }
+
+  auto r = ClampColor((p1 * r1 + p2 * r2) / a);
+  auto g = ClampColor((p1 * g1 + p2 * g2) / a);
+  auto b = ClampColor((p1 * b1 + p2 * b2) / a);
+  return NS_RGBA(r, g, b, NSToIntRound(a * 255));
+}
+
+} // namespace mozilla
+
 // Functions to convert from HSL color space to RGB color space.
 // This is the algorithm described in the CSS3 specification
 
 // helper
 static float
 HSL_HueToRGB(float m1, float m2, float h)
 {
   if (h < 0.0f)

--- a/gfx/src/nsColor.h
+++ b/gfx/src/nsColor.h
@@ -72,16 +72,32 @@ enum class nsHexColorType : uint8_t {
 // This accepts the number of digits specified by aType.
 bool
 NS_HexToRGBA(const nsAString& aBuf, nsHexColorType aType, nscolor* aResult);
 
 // Compose one NS_RGB color onto another. The result is what
 // you get if you draw aFG on top of aBG with operator OVER.
 nscolor NS_ComposeColors(nscolor aBG, nscolor aFG);
 
+namespace mozilla {
+
+inline uint32_t RoundingDivideBy255(uint32_t n)
+{
+  // There is an approximate alternative: ((n << 8) + n + 32896) >> 16
+  // But that is actually slower than this simple expression on a modern
+  // machine with a modern compiler.
+  return (n + 127) / 255;
+}
+
+// Blend one RGBA color with another based on a given ratio.
+// It is a linear interpolation on each channel with alpha premultipled.
+nscolor LinearBlendColors(nscolor aBg, nscolor aFg, uint_fast8_t aFgRatio);
+
+} // namespace mozilla
+
 // Translate a hex string to a color. Return true if it parses ok,
 // otherwise return false.
 // This version accepts 1 to 9 digits (missing digits are 0)
 bool NS_LooseHexToRGB(const nsString& aBuf, nscolor* aResult);
 
 // There is no function to translate a color to a hex string, because
 // the hex-string syntax does not support transparency.