diff --git a/components/layout_2020/flow/mod.rs b/components/layout_2020/flow/mod.rs
index 745f7b57e73..06e7b6a4093 100644
--- a/components/layout_2020/flow/mod.rs
+++ b/components/layout_2020/flow/mod.rs
@@ -410,8 +410,9 @@ fn compute_inline_content_sizes_for_block_level_boxes(
                     &base.style,
                     containing_block,
                     &LogicalVec2::zero(),
-                    false,    /* auto_block_size_stretches_to_containing_block */
-                    false,    /* is_table */
+                    false, /* auto_block_size_stretches_to_containing_block */
+                    false, /* is_table */
+                    !matches!(base.style.pseudo(), Some(PseudoElement::ServoAnonymousBox)),
                     |_| None, /* TODO: support preferred aspect ratios on non-replaced boxes */
                     |constraint_space| {
                         base.inline_content_sizes(layout_context, constraint_space, contents)
diff --git a/components/layout_2020/formatting_contexts.rs b/components/layout_2020/formatting_contexts.rs
index 886328fb304..21383ca40b2 100644
--- a/components/layout_2020/formatting_contexts.rs
+++ b/components/layout_2020/formatting_contexts.rs
@@ -216,6 +216,7 @@ impl IndependentFormattingContext {
             auto_minimum,
             auto_block_size_stretches_to_containing_block,
             is_table,
+            true, /* establishes_containing_block */
             |padding_border_sums| self.preferred_aspect_ratio(padding_border_sums),
             |constraint_space| self.inline_content_sizes(layout_context, constraint_space),
         )
diff --git a/components/layout_2020/geom.rs b/components/layout_2020/geom.rs
index 4df12676c50..65aa70bbf98 100644
--- a/components/layout_2020/geom.rs
+++ b/components/layout_2020/geom.rs
@@ -871,6 +871,14 @@ impl SizeConstraint {
     }
 }
 
+impl From<AuOrAuto> for SizeConstraint {
+    fn from(size: AuOrAuto) -> Self {
+        size.non_auto()
+            .map(SizeConstraint::Definite)
+            .unwrap_or_default()
+    }
+}
+
 #[derive(Clone, Default)]
 pub(crate) struct Sizes {
     /// <https://drafts.csswg.org/css-sizing-3/#preferred-size-properties>
diff --git a/components/layout_2020/sizing.rs b/components/layout_2020/sizing.rs
index 49c9c6aedee..c745081e862 100644
--- a/components/layout_2020/sizing.rs
+++ b/components/layout_2020/sizing.rs
@@ -109,12 +109,14 @@ impl From<Au> for ContentSizes {
     }
 }
 
+#[allow(clippy::too_many_arguments)]
 pub(crate) fn outer_inline(
     style: &ComputedValues,
     containing_block: &IndefiniteContainingBlock,
     auto_minimum: &LogicalVec2<Au>,
     auto_block_size_stretches_to_containing_block: bool,
     is_table: bool,
+    establishes_containing_block: bool,
     get_preferred_aspect_ratio: impl FnOnce(&LogicalVec2<Au>) -> Option<AspectRatio>,
     get_content_size: impl FnOnce(&ConstraintSpace) -> InlineContentSizesResult,
 ) -> InlineContentSizesResult {
@@ -129,28 +131,40 @@ pub(crate) fn outer_inline(
         inline: pbm.padding_border_sums.inline + margin.inline_sum(),
     };
     let content_size = LazyCell::new(|| {
-        let available_block_size = containing_block
-            .size
-            .block
-            .non_auto()
-            .map(|v| Au::zero().max(v - pbm_sums.block));
-        let automatic_size = if content_box_sizes.block.preferred.is_initial() &&
-            auto_block_size_stretches_to_containing_block
-        {
-            depends_on_block_constraints = true;
-            Size::Stretch
+        let constraint_space = if establishes_containing_block {
+            let available_block_size = containing_block
+                .size
+                .block
+                .non_auto()
+                .map(|v| Au::zero().max(v - pbm_sums.block));
+            let automatic_size = if content_box_sizes.block.preferred.is_initial() &&
+                auto_block_size_stretches_to_containing_block
+            {
+                depends_on_block_constraints = true;
+                Size::Stretch
+            } else {
+                Size::FitContent
+            };
+            ConstraintSpace::new(
+                content_box_sizes.block.resolve_extrinsic(
+                    automatic_size,
+                    auto_minimum.block,
+                    available_block_size,
+                ),
+                style.writing_mode,
+                get_preferred_aspect_ratio(&pbm.padding_border_sums),
+            )
         } else {
-            Size::FitContent
+            // This assumes that there is no preferred aspect ratio, or that there is no
+            // block size constraint to be transferred so the ratio is irrelevant.
+            // We only get into here for anonymous blocks, for which the assumption holds.
+            ConstraintSpace::new(
+                containing_block.size.block.into(),
+                containing_block.writing_mode,
+                None,
+            )
         };
-        get_content_size(&ConstraintSpace::new(
-            content_box_sizes.block.resolve_extrinsic(
-                automatic_size,
-                auto_minimum.block,
-                available_block_size,
-            ),
-            style.writing_mode,
-            get_preferred_aspect_ratio(&pbm.padding_border_sums),
-        ))
+        get_content_size(&constraint_space)
     });
     let resolve_non_initial = |inline_size| {
         Some(match inline_size {
diff --git a/tests/wpt/meta/MANIFEST.json b/tests/wpt/meta/MANIFEST.json
index d65a461c843..ddd61bb8185 100644
--- a/tests/wpt/meta/MANIFEST.json
+++ b/tests/wpt/meta/MANIFEST.json
@@ -81587,6 +81587,19 @@
         {}
        ]
       ],
+      "intrinsic-size-with-anonymous-block.html": [
+       "5ddcefbbe1d8e133a80a06e6fb6c3115e886bd89",
+       [
+        null,
+        [
+         [
+          "/css/CSS2/reference/ref-filled-green-100px-square.xht",
+          "=="
+         ]
+        ],
+        {}
+       ]
+      ],
       "intrinsic-size-with-negative-margins.html": [
        "93dfc3dfd770be960eec71bc00082a04e7385e97",
        [
diff --git a/tests/wpt/tests/css/CSS2/normal-flow/intrinsic-size-with-anonymous-block.html b/tests/wpt/tests/css/CSS2/normal-flow/intrinsic-size-with-anonymous-block.html
new file mode 100644
index 00000000000..5ddcefbbe1d
--- /dev/null
+++ b/tests/wpt/tests/css/CSS2/normal-flow/intrinsic-size-with-anonymous-block.html
@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<title>Intrinsic size of an atomic inline with an anonymous block</title>
+<link rel="author" title="Oriol Brufau" href="obrufau@igalia.com">
+<link rel="help" href="https://www.w3.org/TR/CSS22/visudet.html#inlineblock-width">
+<link rel="help" href="https://www.w3.org/TR/CSS22/visudet.html#inline-replaced-width">
+<link rel="help" href="https://www.w3.org/TR/CSS22/visuren.html#anonymous-block-level">
+<link rel="match" href="../reference/ref-filled-green-100px-square.xht">
+<meta name="assert" content="
+  #test contains both inline-level contents (the canvas) and block-level (the <p>).
+  Then the canvas is wrapped inside an anonymous block, but it still resolves its percentage against #test.
+  So the canvas is 100px tall, and thus 100px wide because of its aspect ratio.
+  Therefore #test is 100px wide too.
+">
+<style>
+#test {
+  display: inline-block;
+  height: 100px;
+  background: green;
+}
+#test > canvas {
+  height: 100%;
+  background: red;
+  position: relative;
+  z-index: -1;
+}
+</style>
+
+<p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
+<div id="test">
+  <canvas width="10" height="10"></canvas>
+  <p></p>
+</div>