diff --git a/layout/generic/nsAbsoluteContainingBlock.cpp b/layout/generic/nsAbsoluteContainingBlock.cpp
index 8d929202200f..1df649193863 100644
--- a/layout/generic/nsAbsoluteContainingBlock.cpp
+++ b/layout/generic/nsAbsoluteContainingBlock.cpp
@@ -513,12 +513,12 @@ static nscoord OffsetToAlignedStaticPos(const ReflowInput& aKidReflowInput,
       // absolute containing block.
       alignAreaSize = aAbsPosCBSize.ConvertTo(pcWM, aAbsPosCBWM);
     } else {
-      // The alignment container is a the grid container's padding box (which
-      // we can get by subtracting away its border from frame's size):
+      // The alignment container is a the grid container's content box (which
+      // we can get by subtracting away its border & padding from frame's size):
       alignAreaSize = aPlaceholderContainer->GetLogicalSize(pcWM);
-      LogicalMargin pcBorder =
-          aPlaceholderContainer->GetLogicalUsedBorder(pcWM);
-      alignAreaSize -= pcBorder.Size(pcWM);
+      LogicalMargin pcBorderPadding =
+          aPlaceholderContainer->GetLogicalUsedBorderAndPadding(pcWM);
+      alignAreaSize -= pcBorderPadding.Size(pcWM);
     }
   } else {
     NS_ERROR("Unsupported container for abpsos CSS Box Alignment");
diff --git a/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-align-self-002.html.ini b/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-align-self-002.html.ini
deleted file mode 100644
index 1702b3f178bd..000000000000
--- a/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-align-self-002.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-abspos-staticpos-align-self-002.html]
-  expected: FAIL
diff --git a/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-003.html.ini b/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-003.html.ini
deleted file mode 100644
index e72daaeb0af1..000000000000
--- a/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-003.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-abspos-staticpos-align-self-rtl-003.html]
-  expected: FAIL
diff --git a/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-004.html.ini b/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-004.html.ini
deleted file mode 100644
index eabc393b4b7a..000000000000
--- a/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-004.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-abspos-staticpos-align-self-rtl-004.html]
-  expected: FAIL
diff --git a/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-justify-self-002.html.ini b/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-justify-self-002.html.ini
deleted file mode 100644
index 6a668517f94e..000000000000
--- a/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-justify-self-002.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-abspos-staticpos-justify-self-002.html]
-  expected: FAIL
diff --git a/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-003.html.ini b/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-003.html.ini
deleted file mode 100644
index a30b469a35bf..000000000000
--- a/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-003.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-abspos-staticpos-justify-self-rtl-003.html]
-  expected: FAIL
diff --git a/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-004.html.ini b/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-004.html.ini
deleted file mode 100644
index 6c4fdbbcc1c4..000000000000
--- a/testing/web-platform/meta/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-004.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-abspos-staticpos-justify-self-rtl-004.html]
-  expected: FAIL
diff --git a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-img-last-baseline-002-ref.html b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-img-last-baseline-002-ref.html
index d8652db12474..f35fb48ce18f 100644
--- a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-img-last-baseline-002-ref.html
+++ b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-img-last-baseline-002-ref.html
@@ -33,8 +33,8 @@
     .container > * {
       display: block;
     }
-    .big   .alignEnd    { margin-top:  26px; }
-    .small .alignEnd    { margin-top: -12px; }
+    .big   .alignEnd    { margin-top:  24px; }
+    .small .alignEnd    { margin-top: -14px; }
   </style>
 </head>
 <body>
diff --git a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-last-baseline-002-ref.html b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-last-baseline-002-ref.html
index f1ca180e8283..d02325817748 100644
--- a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-last-baseline-002-ref.html
+++ b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-last-baseline-002-ref.html
@@ -34,8 +34,8 @@
       height: 6px;
       width: 8px;
     }
-    .big   .alignEnd    { margin-top: 26px; }
-    .small .alignEnd    { margin-top: -2px; }
+    .big   .alignEnd    { margin-top: 24px; }
+    .small .alignEnd    { margin-top: -4px; }
   </style>
 </head>
 <body>
diff --git a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-last-baseline-003-ref.html b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-last-baseline-003-ref.html
index c6bdec488c6a..61d6d59fdfe3 100644
--- a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-last-baseline-003-ref.html
+++ b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-last-baseline-003-ref.html
@@ -37,8 +37,8 @@
     .big   > .container > * { margin-left: 14px; }
     .small > .container > * { margin-left: -4px; }
 
-    .big   .alignEnd    { margin-top: 26px; }
-    .small .alignEnd    { margin-top: -2px; }
+    .big   .alignEnd    { margin-top: 24px; }
+    .small .alignEnd    { margin-top: -4px; }
   </style>
 </head>
 <body>
diff --git a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-last-baseline-004-ref.html b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-last-baseline-004-ref.html
index c6bdec488c6a..61d6d59fdfe3 100644
--- a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-last-baseline-004-ref.html
+++ b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-rtl-last-baseline-004-ref.html
@@ -37,8 +37,8 @@
     .big   > .container > * { margin-left: 14px; }
     .small > .container > * { margin-left: -4px; }
 
-    .big   .alignEnd    { margin-top: 26px; }
-    .small .alignEnd    { margin-top: -2px; }
+    .big   .alignEnd    { margin-top: 24px; }
+    .small .alignEnd    { margin-top: -4px; }
   </style>
 </head>
 <body>
diff --git a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-vertWM-last-baseline-003-ref.html b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-vertWM-last-baseline-003-ref.html
index 0623b2d0d857..afa9b43fce3a 100644
--- a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-vertWM-last-baseline-003-ref.html
+++ b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-vertWM-last-baseline-003-ref.html
@@ -35,8 +35,8 @@
       width: 6px;
       height: 8px;
     }
-    .big   .alignEnd    { margin-left: -2px; }
-    .small .alignEnd    { margin-left: -2px; }
+    .big   .alignEnd    { margin-left:  0px; }
+    .small .alignEnd    { margin-left:  0px; }
   </style>
 </head>
 <body>
diff --git a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-vertWM-last-baseline-004-ref.html b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-vertWM-last-baseline-004-ref.html
index 0623b2d0d857..afa9b43fce3a 100644
--- a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-vertWM-last-baseline-004-ref.html
+++ b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-align-self-vertWM-last-baseline-004-ref.html
@@ -35,8 +35,8 @@
       width: 6px;
       height: 8px;
     }
-    .big   .alignEnd    { margin-left: -2px; }
-    .small .alignEnd    { margin-left: -2px; }
+    .big   .alignEnd    { margin-left:  0px; }
+    .small .alignEnd    { margin-left:  0px; }
   </style>
 </head>
 <body>
diff --git a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-img-last-baseline-002-ref.html b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-img-last-baseline-002-ref.html
index bdcd618ebfc0..62c9fe896cd3 100644
--- a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-img-last-baseline-002-ref.html
+++ b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-img-last-baseline-002-ref.html
@@ -33,8 +33,8 @@
     .container > * {
       display: block;
     }
-    .big   .alignEnd    { margin-left: 34px; }
-    .small .alignEnd    { margin-left: -4px; }
+    .big   .alignEnd    { margin-left: 32px; }
+    .small .alignEnd    { margin-left: -6px; }
   </style>
 </head>
 <body>
diff --git a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-last-baseline-002-ref.html b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-last-baseline-002-ref.html
index da33159639a5..299abbae3a5d 100644
--- a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-last-baseline-002-ref.html
+++ b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-last-baseline-002-ref.html
@@ -35,8 +35,8 @@
       width: 6px;
       height: 8px;
     }
-    .big   .alignEnd    { margin-left: 26px; }
-    .small .alignEnd    { margin-left: -2px; }
+    .big   .alignEnd    { margin-left: 24px; }
+    .small .alignEnd    { margin-left: -4px; }
   </style>
 </head>
 <body>
diff --git a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-last-baseline-003-ref.html b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-last-baseline-003-ref.html
index 5f0f6b77fe7b..c70533d399ce 100644
--- a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-last-baseline-003-ref.html
+++ b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-last-baseline-003-ref.html
@@ -35,8 +35,8 @@
       width: 6px;
       height: 8px;
     }
-    .big   .alignEnd    { margin-left: -2px; }
-    .small .alignEnd    { margin-left: -2px; }
+    .big   .alignEnd    { margin-left:  0px; }
+    .small .alignEnd    { margin-left:  0px; }
   </style>
 </head>
 <body>
diff --git a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-last-baseline-004-ref.html b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-last-baseline-004-ref.html
index 5f0f6b77fe7b..c70533d399ce 100644
--- a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-last-baseline-004-ref.html
+++ b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-rtl-last-baseline-004-ref.html
@@ -35,8 +35,8 @@
       width: 6px;
       height: 8px;
     }
-    .big   .alignEnd    { margin-left: -2px; }
-    .small .alignEnd    { margin-left: -2px; }
+    .big   .alignEnd    { margin-left:  0px; }
+    .small .alignEnd    { margin-left:  0px; }
   </style>
 </head>
 <body>
diff --git a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-vertWM-last-baseline-003-ref.html b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-vertWM-last-baseline-003-ref.html
index b82a570fb8e5..83b9391ef317 100644
--- a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-vertWM-last-baseline-003-ref.html
+++ b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-vertWM-last-baseline-003-ref.html
@@ -37,8 +37,8 @@
     .big   > .container > * { margin-left: 14px; }
     .small > .container > * { margin-left: -4px; }
 
-    .big   .alignEnd    { margin-top: 26px; }
-    .small .alignEnd    { margin-top: -2px; }
+    .big   .alignEnd    { margin-top: 24px; }
+    .small .alignEnd    { margin-top: -4px; }
   </style>
 </head>
 <body>
diff --git a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-vertWM-last-baseline-004-ref.html b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-vertWM-last-baseline-004-ref.html
index b82a570fb8e5..83b9391ef317 100644
--- a/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-vertWM-last-baseline-004-ref.html
+++ b/testing/web-platform/tests/css/css-grid/abspos/grid-abspos-staticpos-justify-self-vertWM-last-baseline-004-ref.html
@@ -37,8 +37,8 @@
     .big   > .container > * { margin-left: 14px; }
     .small > .container > * { margin-left: -4px; }
 
-    .big   .alignEnd    { margin-top: 26px; }
-    .small .alignEnd    { margin-top: -2px; }
+    .big   .alignEnd    { margin-top: 24px; }
+    .small .alignEnd    { margin-top: -4px; }
   </style>
 </head>
 <body>