new file mode 100644
--- /dev/null
+++ b/layout/reftests/svg/mask-opacity-01-ref.svg
@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="200" height="200">
+  <rect width="50" height="100" fill="blue" fill-opacity="0.5"/>
+</svg>

new file mode 100644
--- /dev/null
+++ b/layout/reftests/svg/mask-opacity-01.svg
@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="200" height="200">
+  <title>Testcase for the combination of mask, clipPath and opacity</title>
+  <defs>
+    <mask id="m1" maskContentUnits="objectBoundingBox" style="mask-type: alpha;">
+      <rect width="1" height="1" style="stroke:#ffffff; fill: #ffffff; opacity: 0.5;" mask="url(#m2)"/>
+    </mask>
+    <mask id="m2" maskContentUnits="objectBoundingBox" style="mask-type: alpha;">
+      <rect width="0.5" height="1" style="stroke:none; fill: #ffffff"/>
+    </mask>
+  </defs>
+  <rect width="100" height="100" fill="blue" mask="url(#m1)"/>
+</svg>

--- a/layout/reftests/svg/reftest.list
+++ b/layout/reftests/svg/reftest.list
@@ -437,8 +437,10 @@ skip-if(Android) pref(layout.css.mix-ble
 # test case for Fragment URLs
 # https://drafts.csswg.org/css-values/#local-urls
 == use-localRef-marker-01.svg use-localRef-marker-01-ref.svg
 == use-localRef-clipPath-01.svg use-localRef-clipPath-01-ref.svg
 == use-localRef-filter-01.svg use-localRef-filter-01-ref.svg
 == use-localRef-fill-01.svg use-localRef-fill-01-ref.svg
 == use-localRef-stroke-01.svg use-localRef-stroke-01-ref.svg
 == use-localRef-mask-01.svg use-localRef-mask-01-ref.svg
+
+fuzzy(1,5000) == mask-opacity-01.svg mask-opacity-01-ref.svg
\ No newline at end of file