diff --git a/components/layout/block.rs b/components/layout/block.rs
index 2d768b0c55d..4f69a9487d8 100644
--- a/components/layout/block.rs
+++ b/components/layout/block.rs
@@ -809,7 +809,7 @@ impl BlockFlow {
             // At this point, `cur_b` is at the content edge of our box. Now iterate over children.
             let mut floats = self.base.floats.clone();
             let thread_id = self.base.thread_id;
-            let mut had_float_children = false;
+            let (mut had_floated_children, mut had_children_with_clearance) = (false, false);
             for (child_index, kid) in self.base.child_iter_mut().enumerate() {
                 if flow::base(kid).flags.contains(IS_ABSOLUTELY_POSITIONED) {
                     // Assume that the *hypothetical box* for an absolute flow starts immediately
@@ -845,12 +845,12 @@ impl BlockFlow {
                 // before.
                 flow::mut_base(kid).floats = floats.clone();
                 if flow::base(kid).flags.is_float() {
-                    had_float_children = true;
+                    had_floated_children = true;
                     flow::mut_base(kid).position.start.b = cur_b;
                     {
                         let kid_block = kid.as_mut_block();
-                        kid_block.float.as_mut().unwrap().float_ceiling =
-                            margin_collapse_info.current_float_ceiling();
+                        let float_ceiling = margin_collapse_info.current_float_ceiling();
+                        kid_block.float.as_mut().unwrap().float_ceiling = float_ceiling
                     }
                     kid.place_float_if_applicable(layout_context);
 
@@ -875,9 +875,17 @@ impl BlockFlow {
                     kid.assign_block_size_for_inorder_child_if_necessary(layout_context,
                                                                          thread_id);
 
+                if !had_children_with_clearance &&
+                        floats.is_present() &&
+                        (flow::base(kid).flags.contains(CLEARS_LEFT) ||
+                         flow::base(kid).flags.contains(CLEARS_RIGHT)) {
+                    had_children_with_clearance = true
+                }
+
                 // Handle any (possibly collapsed) top margin.
                 let delta = margin_collapse_info.advance_block_start_margin(
-                    &flow::base(kid).collapsible_margins);
+                    &flow::base(kid).collapsible_margins,
+                    !had_children_with_clearance);
                 translate_including_floats(&mut cur_b, delta, &mut floats);
 
                 // Clear past the floats that came in, if necessary.
@@ -935,7 +943,7 @@ impl BlockFlow {
                 &self.fragment,
                 self.base.block_container_explicit_block_size,
                 can_collapse_block_end_margin_with_kids,
-                !had_float_children);
+                !had_floated_children);
             self.base.collapsible_margins = collapsible_margins;
             translate_including_floats(&mut cur_b, delta, &mut floats);
 
diff --git a/components/layout/floats.rs b/components/layout/floats.rs
index a31d7461db0..63fdc6da796 100644
--- a/components/layout/floats.rs
+++ b/components/layout/floats.rs
@@ -413,6 +413,10 @@ impl Floats {
         }
         clearance
     }
+
+    pub fn is_present(&self) -> bool {
+        self.list.is_present()
+    }
 }
 
 /// The speculated inline sizes of floats flowing through or around a flow (depending on whether
diff --git a/components/layout/model.rs b/components/layout/model.rs
index 7bb4cf23cbe..ac592bb4139 100644
--- a/components/layout/model.rs
+++ b/components/layout/model.rs
@@ -18,7 +18,7 @@ use style::values::computed::{BorderRadiusSize, LengthOrPercentageOrAuto};
 use style::values::computed::{LengthOrPercentage, LengthOrPercentageOrNone};
 
 /// A collapsible margin. See CSS 2.1 § 8.3.1.
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Debug)]
 pub struct AdjoiningMargins {
     /// The value of the greatest positive margin.
     pub most_positive: Au,
@@ -61,7 +61,7 @@ impl AdjoiningMargins {
 }
 
 /// Represents the block-start and block-end margins of a flow with collapsible margins. See CSS 2.1 § 8.3.1.
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Debug)]
 pub enum CollapsibleMargins {
     /// Margins may not collapse with this flow.
     None(Au, Au),
@@ -169,12 +169,14 @@ impl MarginCollapseInfo {
                 FinalMarginState::MarginsCollapseThrough => {
                     let advance = self.block_start_margin.collapse();
                     self.margin_in.union(AdjoiningMargins::from_margin(block_end_margin));
-                    (CollapsibleMargins::Collapse(self.block_start_margin, self.margin_in), advance)
+                    (CollapsibleMargins::Collapse(self.block_start_margin, self.margin_in),
+                                                  advance)
                 }
                 FinalMarginState::BottomMarginCollapses => {
                     let advance = self.margin_in.collapse();
                     self.margin_in.union(AdjoiningMargins::from_margin(block_end_margin));
-                    (CollapsibleMargins::Collapse(self.block_start_margin, self.margin_in), advance)
+                    (CollapsibleMargins::Collapse(self.block_start_margin, self.margin_in),
+                                                  advance)
                 }
             }
         } else {
@@ -206,8 +208,14 @@ impl MarginCollapseInfo {
     /// Adds the child's potentially collapsible block-start margin to the current margin state and
     /// advances the Y offset by the appropriate amount to handle that margin. Returns the amount
     /// that should be added to the Y offset during block layout.
-    pub fn advance_block_start_margin(&mut self, child_collapsible_margins: &CollapsibleMargins)
+    pub fn advance_block_start_margin(&mut self,
+                                      child_collapsible_margins: &CollapsibleMargins,
+                                      can_collapse_block_start_margin: bool)
                                       -> Au {
+        if !can_collapse_block_start_margin {
+            self.state = MarginCollapseState::AccumulatingMarginIn
+        }
+
         match (self.state, *child_collapsible_margins) {
             (MarginCollapseState::AccumulatingCollapsibleTopMargin,
              CollapsibleMargins::None(block_start, _)) => {
@@ -226,14 +234,16 @@ impl MarginCollapseInfo {
                 self.margin_in = AdjoiningMargins::new();
                 previous_margin_value + block_start
             }
-            (MarginCollapseState::AccumulatingMarginIn, CollapsibleMargins::Collapse(block_start, _)) => {
+            (MarginCollapseState::AccumulatingMarginIn,
+             CollapsibleMargins::Collapse(block_start, _)) => {
                 self.margin_in.union(block_start);
                 let margin_value = self.margin_in.collapse();
                 self.margin_in = AdjoiningMargins::new();
                 margin_value
             }
             (_, CollapsibleMargins::CollapseThrough(_)) => {
-                // For now, we ignore this; this will be handled by `advance_block-end_margin` below.
+                // For now, we ignore this; this will be handled by `advance_block_end_margin`
+                // below.
                 Au(0)
             }
         }
@@ -274,9 +284,11 @@ impl MarginCollapseInfo {
     }
 }
 
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Debug)]
 pub enum MarginCollapseState {
+    /// We are accumulating margin on the logical top of this flow.
     AccumulatingCollapsibleTopMargin,
+    /// We are accumulating margin between two blocks.
     AccumulatingMarginIn,
 }
 
diff --git a/tests/wpt/css-tests/css21_dev/html4/margin-collapse-018.htm b/tests/wpt/css-tests/css21_dev/html4/margin-collapse-018.htm
index 54628a0f05a..48df2b5173e 100644
--- a/tests/wpt/css-tests/css21_dev/html4/margin-collapse-018.htm
+++ b/tests/wpt/css-tests/css21_dev/html4/margin-collapse-018.htm
@@ -51,4 +51,4 @@
             <div class="class1"></div>
         </div>
     </body>
-</html>
\ No newline at end of file
+</html>
diff --git a/tests/wpt/metadata-css/css-flexbox-1_dev/html/Flexible-order.htm.ini b/tests/wpt/metadata-css/css-flexbox-1_dev/html/Flexible-order.htm.ini
deleted file mode 100644
index 06733df63a9..00000000000
--- a/tests/wpt/metadata-css/css-flexbox-1_dev/html/Flexible-order.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[Flexible-order.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/adjacent-floats-001.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/adjacent-floats-001.htm.ini
deleted file mode 100644
index 751675d3649..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/adjacent-floats-001.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[adjacent-floats-001.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/bidi-010b.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/bidi-010b.htm.ini
new file mode 100644
index 00000000000..b92db65df74
--- /dev/null
+++ b/tests/wpt/metadata-css/css21_dev/html4/bidi-010b.htm.ini
@@ -0,0 +1,4 @@
+[bidi-010b.htm]
+  type: reftest
+  disabled: intermittent failure on reference, starting from PR #10458
+
diff --git a/tests/wpt/metadata-css/css21_dev/html4/border-right-color-175.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/border-right-color-175.htm.ini
deleted file mode 100644
index e01f3c411c0..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/border-right-color-175.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[border-right-color-175.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-002.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-002.htm.ini
deleted file mode 100644
index 289f7e06307..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-002.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[c414-flt-fit-002.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-003.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-003.htm.ini
deleted file mode 100644
index 8fb87cde40d..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-003.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[c414-flt-fit-003.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-004.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-004.htm.ini
deleted file mode 100644
index 8f885773830..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-004.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[c414-flt-fit-004.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-005.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-005.htm.ini
deleted file mode 100644
index 537ca9e2134..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-005.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[c414-flt-fit-005.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-006.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-006.htm.ini
deleted file mode 100644
index b394bb4053d..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/c414-flt-fit-006.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[c414-flt-fit-006.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/clear-002.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/clear-002.htm.ini
deleted file mode 100644
index 5d1ffb0b92a..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/clear-002.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[clear-002.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/clear-float-004.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/clear-float-004.htm.ini
deleted file mode 100644
index b47387c5b20..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/clear-float-004.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[clear-float-004.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/clear-float-005.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/clear-float-005.htm.ini
deleted file mode 100644
index e79aa05629b..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/clear-float-005.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[clear-float-005.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/clear-float-006.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/clear-float-006.htm.ini
deleted file mode 100644
index 9cda771c766..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/clear-float-006.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[clear-float-006.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/clear-float-007.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/clear-float-007.htm.ini
deleted file mode 100644
index be0abf967f9..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/clear-float-007.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[clear-float-007.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/clear-float-008.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/clear-float-008.htm.ini
deleted file mode 100644
index c38ed2c231d..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/clear-float-008.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[clear-float-008.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/clear-float-009.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/clear-float-009.htm.ini
deleted file mode 100644
index 535fccfe97d..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/clear-float-009.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[clear-float-009.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/float-006.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/float-006.htm.ini
deleted file mode 100644
index 751a06666cd..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/float-006.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[float-006.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/float-applies-to-009.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/float-applies-to-009.htm.ini
deleted file mode 100644
index b5f424ebc66..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/float-applies-to-009.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[float-applies-to-009.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/float-applies-to-012.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/float-applies-to-012.htm.ini
deleted file mode 100644
index ab19deefdfd..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/float-applies-to-012.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[float-applies-to-012.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/float-non-replaced-height-001.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/float-non-replaced-height-001.htm.ini
deleted file mode 100644
index b1c53396ecc..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/float-non-replaced-height-001.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[float-non-replaced-height-001.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/floats-003.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/floats-003.htm.ini
deleted file mode 100644
index bb50a94124b..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/floats-003.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[floats-003.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/floats-004.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/floats-004.htm.ini
deleted file mode 100644
index 1301be2990a..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/floats-004.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[floats-004.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/floats-123.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/floats-123.htm.ini
deleted file mode 100644
index 5006229f2c5..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/floats-123.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[floats-123.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/floats-141.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/floats-141.htm.ini
deleted file mode 100644
index 6b5cd007313..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/floats-141.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[floats-141.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/floats-144.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/floats-144.htm.ini
deleted file mode 100644
index 65993a416e3..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/floats-144.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[floats-144.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/floats-145.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/floats-145.htm.ini
index 17b0779acd5..0e215dc3026 100644
--- a/tests/wpt/metadata-css/css21_dev/html4/floats-145.htm.ini
+++ b/tests/wpt/metadata-css/css21_dev/html4/floats-145.htm.ini
@@ -1,3 +1,3 @@
 [floats-145.htm]
   type: reftest
-  expected: FAIL
+  disabled: depends on unspecified line-height behavior, see PR #10458
diff --git a/tests/wpt/metadata-css/css21_dev/html4/floats-146.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/floats-146.htm.ini
deleted file mode 100644
index 2d37eaf361e..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/floats-146.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[floats-146.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/position-absolute-007.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/position-absolute-007.htm.ini
deleted file mode 100644
index cb0a84bc39c..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/position-absolute-007.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[position-absolute-007.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index 3a382bae04b..69c05eaecdc 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -1571,6 +1571,30 @@
             "url": "/_mozilla/css/floated_table_with_margin_a.html"
           }
         ],
+        "css/floats_margin_collapse_a.html": [
+          {
+            "path": "css/floats_margin_collapse_a.html",
+            "references": [
+              [
+                "/_mozilla/css/floats_margin_collapse_ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/css/floats_margin_collapse_a.html"
+          }
+        ],
+        "css/floats_margin_collapse_with_clearance_a.html": [
+          {
+            "path": "css/floats_margin_collapse_with_clearance_a.html",
+            "references": [
+              [
+                "/_mozilla/css/floats_margin_collapse_with_clearance_ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/css/floats_margin_collapse_with_clearance_a.html"
+          }
+        ],
         "css/focus_selector.html": [
           {
             "path": "css/focus_selector.html",
@@ -8161,6 +8185,30 @@
           "url": "/_mozilla/css/floated_table_with_margin_a.html"
         }
       ],
+      "css/floats_margin_collapse_a.html": [
+        {
+          "path": "css/floats_margin_collapse_a.html",
+          "references": [
+            [
+              "/_mozilla/css/floats_margin_collapse_ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/css/floats_margin_collapse_a.html"
+        }
+      ],
+      "css/floats_margin_collapse_with_clearance_a.html": [
+        {
+          "path": "css/floats_margin_collapse_with_clearance_a.html",
+          "references": [
+            [
+              "/_mozilla/css/floats_margin_collapse_with_clearance_ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/css/floats_margin_collapse_with_clearance_a.html"
+        }
+      ],
       "css/focus_selector.html": [
         {
           "path": "css/focus_selector.html",
diff --git a/tests/wpt/mozilla/tests/css/floats_margin_collapse_a.html b/tests/wpt/mozilla/tests/css/floats_margin_collapse_a.html
new file mode 100644
index 00000000000..06d0b6a386b
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/floats_margin_collapse_a.html
@@ -0,0 +1,42 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<link rel="match" href="floats_margin_collapse_ref.html">
+<!--
+From https://github.com/servo/servo/pull/10458:
+
+https://drafts.csswg.org/css2/visuren.html#float-rules
+
+    A floating box's outer top may not be higher than the top of its containing block. When the float occurs between two collapsing margins, the float is positioned as if it had an otherwise empty anonymous block parent taking part in the flow. The position of such a parent is defined by the rules in the section on margin collapsing.
+
+#a is not "in the flow" since it’s a float, which makes #b the first in-flow child of <body>, which makes the top margins of <body> and #b "adjoining" (even if the former is zero) which makes them collapse.
+
+So the quoted rule applies and there’s an hypothetical block around #a. Since that blocks has no in-flow content, margins collapse through it.
+
+Then I believe this rule applies:
+
+https://drafts.csswg.org/css2/box.html#collapsed-through
+
+    If the element's margins are collapsed with its parent's top margin, the top border edge of the box is defined to be the same as the parent's.
+
+The top border edge of <body> is below its top margin, which is collapsed with that of #b.
+-->
+<style>
+html, body {
+    margin: 0;
+}
+div {
+    width: 32px;
+    height: 32px;
+}
+#a {
+    float: left;
+    background: green;
+}
+#b {
+    background: blue;
+    margin-top: 32px;
+}
+</style>
+<div id=a></div>
+<div id=b></div>
+
diff --git a/tests/wpt/mozilla/tests/css/floats_margin_collapse_ref.html b/tests/wpt/mozilla/tests/css/floats_margin_collapse_ref.html
new file mode 100644
index 00000000000..164370a58f7
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/floats_margin_collapse_ref.html
@@ -0,0 +1,19 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<style>
+html, body {
+    margin: 0;
+}
+div {
+    width: 32px;
+    height: 32px;
+    position: absolute;
+    left: 0;
+}
+#a {
+    top: 32px;
+    background: green;
+}
+</style>
+<div id=a></div>
+
diff --git a/tests/wpt/mozilla/tests/css/floats_margin_collapse_with_clearance_a.html b/tests/wpt/mozilla/tests/css/floats_margin_collapse_with_clearance_a.html
new file mode 100644
index 00000000000..a2745f796cf
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/floats_margin_collapse_with_clearance_a.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title></title>
+<link rel="match" href="floats_margin_collapse_with_clearance_ref.html">
+<!--
+From https://github.com/servo/servo/pull/10458:
+
+https://drafts.csswg.org/css2/visuren.html#float-rules
+
+    A floating box's outer top may not be higher than the top of its containing block. When the float occurs between two collapsing margins, the float is positioned as if it had an otherwise empty anonymous block parent taking part in the flow. The position of such a parent is defined by the rules in the section on margin collapsing.
+
+#a is not "in the flow" since it’s a float, which makes #b the first in-flow child of <body>, which makes the top margins of <body> and #b "adjoining" (even if the former is zero) which makes them collapse.
+
+So the quoted rule applies and there’s an hypothetical block around #a. Since that blocks has no in-flow content, margins collapse through it.
+
+Then I believe this rule applies:
+
+https://drafts.csswg.org/css2/box.html#collapsed-through
+
+    If the element's margins are collapsed with its parent's top margin, the top border edge of the box is defined to be the same as the parent's.
+
+The top border edge of <body> is below its top margin, which is collapsed with that of #b.
+-->
+<style>
+html, body {
+    margin: 0;
+}
+div {
+    width: 32px;
+    height: 32px;
+}
+#a {
+    float: left;
+    background: green;
+}
+#b {
+    background: blue;
+    margin-top: 32px;
+    clear: left;
+}
+</style>
+<div id=a></div>
+<div id=b></div>
+
+
diff --git a/tests/wpt/mozilla/tests/css/floats_margin_collapse_with_clearance_ref.html b/tests/wpt/mozilla/tests/css/floats_margin_collapse_with_clearance_ref.html
new file mode 100644
index 00000000000..58fb43b5105
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/floats_margin_collapse_with_clearance_ref.html
@@ -0,0 +1,25 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<style>
+html, body {
+    margin: 0;
+}
+div {
+    width: 32px;
+    height: 32px;
+    position: absolute;
+    left: 0;
+}
+#a {
+    background: green;
+    top: 0;
+}
+#b {
+    background: blue;
+    top: 32px;
+}
+</style>
+<div id=a></div>
+<div id=b></div>
+
+