diff --git a/Libraries/LibWeb/CSS/ResolvedCSSStyleDeclaration.cpp b/Libraries/LibWeb/CSS/ResolvedCSSStyleDeclaration.cpp
index a3292c10630..244bfe251af 100644
--- a/Libraries/LibWeb/CSS/ResolvedCSSStyleDeclaration.cpp
+++ b/Libraries/LibWeb/CSS/ResolvedCSSStyleDeclaration.cpp
@@ -241,7 +241,7 @@ RefPtr<CSSStyleValue const> ResolvedCSSStyleDeclaration::style_value_for_propert
         return LengthStyleValue::create(Length::make_px(layout_node.computed_values().line_height()));
     }
 
-        // FIXME: -> block-size
+        // -> block-size
         // -> height
         // FIXME: -> inline-size
         // -> margin-block-end
@@ -264,6 +264,13 @@ RefPtr<CSSStyleValue const> ResolvedCSSStyleDeclaration::style_value_for_propert
         // If the property applies to the element or pseudo-element and the resolved value of the
         // display property is not none or contents, then the resolved value is the used value.
         // Otherwise the resolved value is the computed value.
+    case PropertyID::BlockSize: {
+        auto writing_mode = layout_node.computed_values().writing_mode();
+        auto is_vertically_oriented = first_is_one_of(writing_mode, WritingMode::VerticalLr, WritingMode::VerticalRl);
+        if (is_vertically_oriented)
+            return style_value_for_property(layout_node, PropertyID::Width);
+        return style_value_for_property(layout_node, PropertyID::Height);
+    }
     case PropertyID::Height: {
         auto maybe_used_height = used_value_for_property([](auto const& paintable_box) { return paintable_box.content_height(); });
         if (maybe_used_height.has_value())
diff --git a/Tests/LibWeb/Text/expected/css/CSSStyleDeclaration-all-supported-properties-and-default-values.txt b/Tests/LibWeb/Text/expected/css/CSSStyleDeclaration-all-supported-properties-and-default-values.txt
index b4108636ba4..8b3ec729747 100644
--- a/Tests/LibWeb/Text/expected/css/CSSStyleDeclaration-all-supported-properties-and-default-values.txt
+++ b/Tests/LibWeb/Text/expected/css/CSSStyleDeclaration-all-supported-properties-and-default-values.txt
@@ -183,8 +183,8 @@ All supported properties and their default values exposed from CSSStyleDeclarati
 'background-repeat': 'repeat'
 'backgroundSize': 'auto auto'
 'background-size': 'auto auto'
-'blockSize': 'auto'
-'block-size': 'auto'
+'blockSize': '0px'
+'block-size': '0px'
 'border': 'medium none rgb(0, 0, 0)'
 'borderBottom': 'medium none rgb(0, 0, 0)'
 'border-bottom': 'medium none rgb(0, 0, 0)'
diff --git a/Tests/LibWeb/Text/expected/css/getComputedStyle-print-all.txt b/Tests/LibWeb/Text/expected/css/getComputedStyle-print-all.txt
index e826e6fb3a3..6eca7cfd63f 100644
--- a/Tests/LibWeb/Text/expected/css/getComputedStyle-print-all.txt
+++ b/Tests/LibWeb/Text/expected/css/getComputedStyle-print-all.txt
@@ -83,7 +83,7 @@ background-position-x: 0%
 background-position-y: 0%
 background-repeat: repeat
 background-size: auto auto
-block-size: auto
+block-size: 1190px
 border-bottom-color: rgb(0, 0, 0)
 border-bottom-left-radius: 0px
 border-bottom-right-radius: 0px
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-logical/parsing/block-size-computed.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-logical/parsing/block-size-computed.txt
new file mode 100644
index 00000000000..c948e644281
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-logical/parsing/block-size-computed.txt
@@ -0,0 +1,12 @@
+Harness status: OK
+
+Found 7 tests
+
+7 Pass
+Pass	Property block-size value 'auto'
+Pass	Property block-size value '10px'
+Pass	Property block-size value '20%'
+Pass	Property block-size value 'calc(0.5em + 10px)'
+Pass	Property block-size value 'calc(-0.5em + 10px)'
+Pass	Property block-size value 'min-content'
+Pass	Property block-size value 'max-content'
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-logical/parsing/block-size-computed.html b/Tests/LibWeb/Text/input/wpt-import/css/css-logical/parsing/block-size-computed.html
new file mode 100644
index 00000000000..68ac0bb4561
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-logical/parsing/block-size-computed.html
@@ -0,0 +1,44 @@
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>CSS Logical Properties and Values: getComputedStyle().blockSize</title>
+<link rel="help" href="https://drafts.csswg.org/css-logical-1/#dimension-properties">
+<meta name="assert" content="block-size computed value is an absolute length.">
+<script src="../../../resources/testharness.js"></script>
+<script src="../../../resources/testharnessreport.js"></script>
+<script src="../../../css/support/computed-testcommon.js"></script>
+<style>
+  #parent {
+    height: 300px;
+  }
+  #target {
+    width: 0px;
+    height: 0px;
+    font-size: 40px;
+  }
+  #child {
+    height: 80px;
+  }
+</style>
+</head>
+<body>
+<div id="parent">
+  <div id="target">
+    <div id="child">
+    </div>
+  </div>
+</div>
+<script>
+test_computed_value("block-size", "auto", "80px"); // child height
+
+test_computed_value("block-size", "10px");
+test_computed_value("block-size", "20%", "60px");
+test_computed_value("block-size", "calc(0.5em + 10px)", "30px");
+test_computed_value("block-size", "calc(-0.5em + 10px)", "0px");
+
+test_computed_value("block-size", "min-content", "80px"); // child height
+test_computed_value("block-size", "max-content", "80px");
+</script>
+</body>
+</html>