diff --git a/components/layout/block.rs b/components/layout/block.rs
index fa5790306b5..090672c00fb 100644
--- a/components/layout/block.rs
+++ b/components/layout/block.rs
@@ -1650,10 +1650,18 @@ impl BlockFlow {
                 // page set `max-width` in order to avoid hitting floats. The search box on Google
                 // SERPs falls into this category.)
                 if self.fragment.style.max_inline_size() == LengthOrPercentageOrNone::None {
+                    let speculated_left_float_size =
+                        max(Au(0),
+                            self.base.speculated_float_placement_in.left -
+                            self.fragment.margin.inline_start);
+                    let speculated_right_float_size =
+                        max(Au(0),
+                            self.base.speculated_float_placement_in.right -
+                            self.fragment.margin.inline_end);
                     self.fragment.border_box.size.inline =
                         self.fragment.border_box.size.inline -
-                        self.base.speculated_float_placement_in.left -
-                        self.base.speculated_float_placement_in.right;
+                        speculated_left_float_size -
+                        speculated_right_float_size;
                 }
             }
             FormattingContextType::None | FormattingContextType::Other => {}
diff --git a/components/layout/floats.rs b/components/layout/floats.rs
index 9a5d1521d58..9fee4db43ca 100644
--- a/components/layout/floats.rs
+++ b/components/layout/floats.rs
@@ -475,20 +475,18 @@ impl SpeculatedFloatPlacement {
             let block_flow = flow.as_block();
             if block_flow.formatting_context_type() != FormattingContextType::None {
                 *self = block_flow.base.speculated_float_placement_in;
-            }
-
-            if self.left > Au(0) || self.right > Au(0) {
-                let speculated_inline_content_edge_offsets =
-                    block_flow.fragment.guess_inline_content_edge_offsets();
-                if self.left > Au(0) && speculated_inline_content_edge_offsets.start > Au(0) {
-                    self.left = self.left + speculated_inline_content_edge_offsets.start
+            } else {
+                if self.left > Au(0) || self.right > Au(0) {
+                    let speculated_inline_content_edge_offsets =
+                        block_flow.fragment.guess_inline_content_edge_offsets();
+                    if self.left > Au(0) && speculated_inline_content_edge_offsets.start > Au(0) {
+                        self.left = self.left + speculated_inline_content_edge_offsets.start
+                    }
+                    if self.right > Au(0) && speculated_inline_content_edge_offsets.end > Au(0) {
+                        self.right = self.right + speculated_inline_content_edge_offsets.end
+                    }
                 }
-                if self.right > Au(0) && speculated_inline_content_edge_offsets.end > Au(0) {
-                    self.right = self.right + speculated_inline_content_edge_offsets.end
-                }
-            }
 
-            if block_flow.formatting_context_type() == FormattingContextType::None {
                 self.left = max(self.left, block_flow.base.speculated_float_placement_in.left);
                 self.right = max(self.right, block_flow.base.speculated_float_placement_in.right);
             }
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index aacefe74da3..d81335ad928 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -616,6 +616,18 @@
             "url": "/_mozilla/css/block_formatting_context_margin_collapse_a.html"
           }
         ],
+        "css/block_formatting_context_margin_inout_a.html": [
+          {
+            "path": "css/block_formatting_context_margin_inout_a.html",
+            "references": [
+              [
+                "/_mozilla/css/block_formatting_context_margin_inout_ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/css/block_formatting_context_margin_inout_a.html"
+          }
+        ],
         "css/block_formatting_context_max_width_a.html": [
           {
             "path": "css/block_formatting_context_max_width_a.html",
@@ -7410,6 +7422,18 @@
           "url": "/_mozilla/css/block_formatting_context_margin_collapse_a.html"
         }
       ],
+      "css/block_formatting_context_margin_inout_a.html": [
+        {
+          "path": "css/block_formatting_context_margin_inout_a.html",
+          "references": [
+            [
+              "/_mozilla/css/block_formatting_context_margin_inout_ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/css/block_formatting_context_margin_inout_a.html"
+        }
+      ],
       "css/block_formatting_context_max_width_a.html": [
         {
           "path": "css/block_formatting_context_max_width_a.html",
diff --git a/tests/wpt/mozilla/tests/css/block_formatting_context_margin_inout_a.html b/tests/wpt/mozilla/tests/css/block_formatting_context_margin_inout_a.html
new file mode 100644
index 00000000000..a7e6f0cd69e
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/block_formatting_context_margin_inout_a.html
@@ -0,0 +1,24 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<link rel="match" href="block_formatting_context_margin_inout_ref.html">
+<style>
+html, body {
+    margin: 0;
+}
+div {
+    float: right;
+    width: 300px;
+    height: 32px;
+    background: green;
+}
+section {
+    margin-right: 300px;
+    overflow: hidden;
+    height: 64px;
+    background: blue;
+}
+</style>
+<div></div>
+<section></section>
+<section></section>
+
diff --git a/tests/wpt/mozilla/tests/css/block_formatting_context_margin_inout_ref.html b/tests/wpt/mozilla/tests/css/block_formatting_context_margin_inout_ref.html
new file mode 100644
index 00000000000..c333a800cc4
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/block_formatting_context_margin_inout_ref.html
@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<style>
+html, body {
+    margin: 0;
+}
+div, section {
+    position: absolute;
+}
+div {
+    width: 300px;
+    height: 32px;
+    right: 0;
+    background: green;
+}
+section {
+    left: 0;
+    right: 300px;
+    height: 64px;
+    background: blue;
+}
+#a {
+    top: 0;
+}
+#b {
+    top: 64px;
+}
+</style>
+<div></div>
+<section id=a></section>
+<section id=b></section>
+