diff --git a/Libraries/LibWeb/SVG/SVGAnimatedLength.cpp b/Libraries/LibWeb/SVG/SVGAnimatedLength.cpp
index 0eacbbddd0d..33585e41064 100644
--- a/Libraries/LibWeb/SVG/SVGAnimatedLength.cpp
+++ b/Libraries/LibWeb/SVG/SVGAnimatedLength.cpp
@@ -14,16 +14,21 @@ GC_DEFINE_ALLOCATOR(SVGAnimatedLength);
 
 GC::Ref<SVGAnimatedLength> SVGAnimatedLength::create(JS::Realm& realm, GC::Ref<SVGLength> base_val, GC::Ref<SVGLength> anim_val)
 {
-    return realm.create<SVGAnimatedLength>(realm, move(base_val), move(anim_val));
+    return realm.create<SVGAnimatedLength>(realm, base_val, anim_val);
 }
 
 SVGAnimatedLength::SVGAnimatedLength(JS::Realm& realm, GC::Ref<SVGLength> base_val, GC::Ref<SVGLength> anim_val)
     : PlatformObject(realm)
-    , m_base_val(move(base_val))
-    , m_anim_val(move(anim_val))
+    , m_base_val(base_val)
+    , m_anim_val(anim_val)
 {
-    // The object referenced by animVal will always be distinct from the one referenced by baseVal, even when the attribute is not animated.
+    // The object referenced by animVal will always be distinct from the one referenced by baseVal, even when the
+    // attribute is not animated.
     VERIFY(m_base_val.ptr() != m_anim_val.ptr());
+
+    // https://svgwg.org/svg2-draft/types.html#InterfaceSVGLength
+    // SVGLength objects reflected through the animVal IDL attribute are always read only.
+    VERIFY(m_anim_val->read_only() == SVGLength::ReadOnly::Yes);
 }
 
 SVGAnimatedLength::~SVGAnimatedLength() = default;
@@ -34,7 +39,7 @@ void SVGAnimatedLength::initialize(JS::Realm& realm)
     Base::initialize(realm);
 }
 
-void SVGAnimatedLength::visit_edges(Cell::Visitor& visitor)
+void SVGAnimatedLength::visit_edges(Visitor& visitor)
 {
     Base::visit_edges(visitor);
     visitor.visit(m_base_val);
diff --git a/Libraries/LibWeb/SVG/SVGElement.cpp b/Libraries/LibWeb/SVG/SVGElement.cpp
index bd584b86ce7..daf7317989a 100644
--- a/Libraries/LibWeb/SVG/SVGElement.cpp
+++ b/Libraries/LibWeb/SVG/SVGElement.cpp
@@ -1,6 +1,7 @@
 /*
  * Copyright (c) 2020, Matthew Olsson <mattco@serenityos.org>
  * Copyright (c) 2023, Preston Taylor <95388976+PrestonLTaylor@users.noreply.github.com>
+ * Copyright (c) 2025, Jelle Raaijmakers <jelle@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -298,17 +299,28 @@ GC::Ptr<SVGElement> SVGElement::viewport_element()
     return nullptr;
 }
 
+GC::Ref<SVGAnimatedLength> SVGElement::fake_animated_length_fixme() const
+{
+    // FIXME: All callers of this method must implement their animated length correctly.
+    auto base_length = SVGLength::create(realm(), 0, 0, SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, 0, SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
+}
+
 GC::Ref<SVGAnimatedLength> SVGElement::svg_animated_length_for_property(CSS::PropertyID property) const
 {
     // FIXME: Create a proper animated value when animations are supported.
-    auto make_length = [&] {
+    auto make_length = [&](SVGLength::ReadOnly read_only) {
         if (auto const computed_properties = this->computed_properties()) {
             if (auto length = computed_properties->length_percentage(property); length.has_value())
-                return SVGLength::from_length_percentage(realm(), *length);
+                return SVGLength::from_length_percentage(realm(), *length, read_only);
         }
-        return SVGLength::create(realm(), 0, 0.0f);
+        return SVGLength::create(realm(), 0, 0, read_only);
     };
-    return SVGAnimatedLength::create(realm(), make_length(), make_length());
+    return SVGAnimatedLength::create(
+        realm(),
+        make_length(SVGLength::ReadOnly::No),
+        make_length(SVGLength::ReadOnly::Yes));
 }
 
 }
diff --git a/Libraries/LibWeb/SVG/SVGElement.h b/Libraries/LibWeb/SVG/SVGElement.h
index 781e70f583f..9ecb4cec318 100644
--- a/Libraries/LibWeb/SVG/SVGElement.h
+++ b/Libraries/LibWeb/SVG/SVGElement.h
@@ -30,6 +30,7 @@ public:
     bool should_include_in_accessibility_tree() const;
     virtual Optional<ARIA::Role> default_role() const override;
 
+    GC::Ref<SVGAnimatedLength> fake_animated_length_fixme() const;
     GC::Ref<SVGAnimatedLength> svg_animated_length_for_property(CSS::PropertyID) const;
 
     virtual bool is_presentational_hint(FlyString const&) const override;
diff --git a/Libraries/LibWeb/SVG/SVGEllipseElement.cpp b/Libraries/LibWeb/SVG/SVGEllipseElement.cpp
index 6846a7ef45e..16fc48cbabd 100644
--- a/Libraries/LibWeb/SVG/SVGEllipseElement.cpp
+++ b/Libraries/LibWeb/SVG/SVGEllipseElement.cpp
@@ -81,9 +81,9 @@ GC::Ref<SVGAnimatedLength> SVGEllipseElement::cx() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_center_x.value_or(0));
-    auto anim_length = SVGLength::create(realm(), 0, m_center_x.value_or(0));
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_center_x.value_or(0), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_center_x.value_or(0), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 // https://www.w3.org/TR/SVG11/shapes.html#EllipseElementCYAttribute
@@ -91,9 +91,9 @@ GC::Ref<SVGAnimatedLength> SVGEllipseElement::cy() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_center_y.value_or(0));
-    auto anim_length = SVGLength::create(realm(), 0, m_center_y.value_or(0));
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_center_y.value_or(0), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_center_y.value_or(0), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 // https://www.w3.org/TR/SVG11/shapes.html#EllipseElementRXAttribute
@@ -101,9 +101,9 @@ GC::Ref<SVGAnimatedLength> SVGEllipseElement::rx() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_radius_x.value_or(0));
-    auto anim_length = SVGLength::create(realm(), 0, m_radius_x.value_or(0));
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_radius_x.value_or(0), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_radius_x.value_or(0), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 // https://www.w3.org/TR/SVG11/shapes.html#EllipseElementRYAttribute
@@ -111,9 +111,9 @@ GC::Ref<SVGAnimatedLength> SVGEllipseElement::ry() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_radius_y.value_or(0));
-    auto anim_length = SVGLength::create(realm(), 0, m_radius_y.value_or(0));
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_radius_y.value_or(0), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_radius_y.value_or(0), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 }
diff --git a/Libraries/LibWeb/SVG/SVGForeignObjectElement.cpp b/Libraries/LibWeb/SVG/SVGForeignObjectElement.cpp
index 4c089704fe2..a26abc0f696 100644
--- a/Libraries/LibWeb/SVG/SVGForeignObjectElement.cpp
+++ b/Libraries/LibWeb/SVG/SVGForeignObjectElement.cpp
@@ -32,10 +32,10 @@ void SVGForeignObjectElement::initialize(JS::Realm& realm)
     Base::initialize(realm);
 
     // FIXME: These never actually get updated!
-    m_x = SVGAnimatedLength::create(realm, SVGLength::create(realm, 0, 0), SVGLength::create(realm, 0, 0));
-    m_y = SVGAnimatedLength::create(realm, SVGLength::create(realm, 0, 0), SVGLength::create(realm, 0, 0));
-    m_width = SVGAnimatedLength::create(realm, SVGLength::create(realm, 0, 0), SVGLength::create(realm, 0, 0));
-    m_height = SVGAnimatedLength::create(realm, SVGLength::create(realm, 0, 0), SVGLength::create(realm, 0, 0));
+    m_x = fake_animated_length_fixme();
+    m_y = fake_animated_length_fixme();
+    m_width = fake_animated_length_fixme();
+    m_height = fake_animated_length_fixme();
 }
 
 void SVGForeignObjectElement::visit_edges(Cell::Visitor& visitor)
diff --git a/Libraries/LibWeb/SVG/SVGImageElement.cpp b/Libraries/LibWeb/SVG/SVGImageElement.cpp
index f15723fcedd..cdc05fcdbd0 100644
--- a/Libraries/LibWeb/SVG/SVGImageElement.cpp
+++ b/Libraries/LibWeb/SVG/SVGImageElement.cpp
@@ -81,10 +81,8 @@ void SVGImageElement::attribute_changed(FlyString const& name, Optional<String>
 // https://svgwg.org/svg2-draft/embedded.html#__svg__SVGImageElement__x
 GC::Ref<SVG::SVGAnimatedLength> SVGImageElement::x()
 {
-    if (!m_x) {
-        auto& realm = this->realm();
-        m_x = SVGAnimatedLength::create(realm, SVGLength::create(realm, 0, 0), SVGLength::create(realm, 0, 0));
-    }
+    if (!m_x)
+        m_x = fake_animated_length_fixme();
 
     return *m_x;
 }
@@ -92,10 +90,8 @@ GC::Ref<SVG::SVGAnimatedLength> SVGImageElement::x()
 // https://svgwg.org/svg2-draft/embedded.html#__svg__SVGImageElement__y
 GC::Ref<SVG::SVGAnimatedLength> SVGImageElement::y()
 {
-    if (!m_y) {
-        auto& realm = this->realm();
-        m_y = SVGAnimatedLength::create(realm, SVGLength::create(realm, 0, 0), SVGLength::create(realm, 0, 0));
-    }
+    if (!m_y)
+        m_y = fake_animated_length_fixme();
 
     return *m_y;
 }
@@ -105,7 +101,10 @@ GC::Ref<SVG::SVGAnimatedLength> SVGImageElement::width()
 {
     if (!m_width) {
         auto& realm = this->realm();
-        m_width = SVGAnimatedLength::create(realm, SVGLength::create(realm, 0, intrinsic_width().value_or(0).to_double()), SVGLength::create(realm, 0, 0));
+        m_width = SVGAnimatedLength::create(
+            realm,
+            SVGLength::create(realm, 0, intrinsic_width().value_or(0).to_double(), SVGLength::ReadOnly::No),
+            SVGLength::create(realm, 0, 0, SVGLength::ReadOnly::Yes));
     }
 
     return *m_width;
@@ -116,7 +115,10 @@ GC::Ref<SVG::SVGAnimatedLength> SVGImageElement::height()
 {
     if (!m_height) {
         auto& realm = this->realm();
-        m_height = SVGAnimatedLength::create(realm, SVGLength::create(realm, 0, intrinsic_height().value_or(0).to_double()), SVGLength::create(realm, 0, 0));
+        m_height = SVGAnimatedLength::create(
+            realm,
+            SVGLength::create(realm, 0, intrinsic_height().value_or(0).to_double(), SVGLength::ReadOnly::No),
+            SVGLength::create(realm, 0, 0, SVGLength::ReadOnly::Yes));
     }
 
     return *m_height;
diff --git a/Libraries/LibWeb/SVG/SVGLength.cpp b/Libraries/LibWeb/SVG/SVGLength.cpp
index ffd5b4dd03a..c4d929b30b4 100644
--- a/Libraries/LibWeb/SVG/SVGLength.cpp
+++ b/Libraries/LibWeb/SVG/SVGLength.cpp
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2022, Tim Flynn <trflynn89@serenityos.org>
+ * Copyright (c) 2025, Jelle Raaijmakers <jelle@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -13,19 +14,20 @@ namespace Web::SVG {
 
 GC_DEFINE_ALLOCATOR(SVGLength);
 
-GC::Ref<SVGLength> SVGLength::create(JS::Realm& realm, u8 unit_type, float value)
+GC::Ref<SVGLength> SVGLength::create(JS::Realm& realm, u8 unit_type, float value, ReadOnly read_only)
 {
-    return realm.create<SVGLength>(realm, unit_type, value);
+    return realm.create<SVGLength>(realm, unit_type, value, read_only);
 }
 
-GC::Ref<SVGLength> SVGLength::from_length_percentage(JS::Realm& realm, CSS::LengthPercentage const& length_percentage)
+GC::Ref<SVGLength> SVGLength::from_length_percentage(JS::Realm& realm, CSS::LengthPercentage const& length_percentage,
+    ReadOnly read_only)
 {
     // FIXME: We can't tell if a CSS::LengthPercentage was a unitless length.
     (void)SVG_LENGTHTYPE_NUMBER;
     if (length_percentage.is_percentage())
-        return SVGLength::create(realm, SVG_LENGTHTYPE_PERCENTAGE, length_percentage.percentage().value());
+        return create(realm, SVG_LENGTHTYPE_PERCENTAGE, length_percentage.percentage().value(), read_only);
     if (length_percentage.is_length())
-        return SVGLength::create(
+        return create(
             realm, [&] {
                 switch (length_percentage.length().type()) {
                 case CSS::Length::Type::Em:
@@ -48,14 +50,15 @@ GC::Ref<SVGLength> SVGLength::from_length_percentage(JS::Realm& realm, CSS::Leng
                     return SVG_LENGTHTYPE_UNKNOWN;
                 }
             }(),
-            length_percentage.length().raw_value());
-    return SVGLength::create(realm, SVG_LENGTHTYPE_UNKNOWN, 0);
+            length_percentage.length().raw_value(), read_only);
+    return create(realm, SVG_LENGTHTYPE_UNKNOWN, 0, read_only);
 }
 
-SVGLength::SVGLength(JS::Realm& realm, u8 unit_type, float value)
+SVGLength::SVGLength(JS::Realm& realm, u8 unit_type, float value, ReadOnly read_only)
     : PlatformObject(realm)
-    , m_unit_type(unit_type)
     , m_value(value)
+    , m_unit_type(unit_type)
+    , m_read_only(read_only)
 {
 }
 
@@ -67,11 +70,21 @@ void SVGLength::initialize(JS::Realm& realm)
 
 SVGLength::~SVGLength() = default;
 
-// https://www.w3.org/TR/SVG11/types.html#__svg__SVGLength__value
+// https://svgwg.org/svg2-draft/types.html#__svg__SVGLength__value
 WebIDL::ExceptionOr<void> SVGLength::set_value(float value)
 {
-    // FIXME: Raise an exception if this <length> is read-only.
+    // 1. If the SVGLength object is read only, then throw a NoModificationAllowedError.
+    if (m_read_only == ReadOnly::Yes)
+        return WebIDL::NoModificationAllowedError::create(realm(), "Cannot modify value of read-only SVGLength"_utf16);
+
+    // 2. Let value be the value being assigned to value.
+    // 3. Set the SVGLength's value to a <number> whose value is value.
     m_value = value;
+    m_unit_type = SVG_LENGTHTYPE_NUMBER;
+
+    // FIXME: 4. If the SVGLength reflects the base value of a reflected attribute, reflects a presentation attribute, or
+    //    reflects an element of the base value of a reflected attribute, then reserialize the reflected attribute.
+
     return {};
 }
 
diff --git a/Libraries/LibWeb/SVG/SVGLength.h b/Libraries/LibWeb/SVG/SVGLength.h
index 2bba7f89096..66f90fc775d 100644
--- a/Libraries/LibWeb/SVG/SVGLength.h
+++ b/Libraries/LibWeb/SVG/SVGLength.h
@@ -30,23 +30,32 @@ public:
     static constexpr unsigned short SVG_LENGTHTYPE_PT = 9;
     static constexpr unsigned short SVG_LENGTHTYPE_PC = 10;
 
-    [[nodiscard]] static GC::Ref<SVGLength> create(JS::Realm&, u8 unit_type, float value);
-    virtual ~SVGLength() override;
+    enum class ReadOnly : u8 {
+        Yes,
+        No,
+    };
 
-    u8 unit_type() const { return m_unit_type; }
+    [[nodiscard]] static GC::Ref<SVGLength> create(JS::Realm&, u8 unit_type, float value, ReadOnly);
+    virtual ~SVGLength() override;
 
     float value() const { return m_value; }
     WebIDL::ExceptionOr<void> set_value(float value);
 
-    [[nodiscard]] static GC::Ref<SVGLength> from_length_percentage(JS::Realm&, CSS::LengthPercentage const&);
+    u8 unit_type() const { return m_unit_type; }
+    ReadOnly read_only() const { return m_read_only; }
+
+    [[nodiscard]] static GC::Ref<SVGLength> from_length_percentage(JS::Realm&, CSS::LengthPercentage const&, ReadOnly);
 
 private:
-    SVGLength(JS::Realm&, u8 unit_type, float value);
+    SVGLength(JS::Realm&, u8 unit_type, float value, ReadOnly);
 
     virtual void initialize(JS::Realm&) override;
 
-    u8 m_unit_type { 0 };
     float m_value { 0 };
+    u8 m_unit_type { 0 };
+
+    // https://svgwg.org/svg2-draft/types.html#ReadOnlyLength
+    ReadOnly m_read_only;
 };
 
 }
diff --git a/Libraries/LibWeb/SVG/SVGLineElement.cpp b/Libraries/LibWeb/SVG/SVGLineElement.cpp
index fc2370c6b3a..3b0010144e6 100644
--- a/Libraries/LibWeb/SVG/SVGLineElement.cpp
+++ b/Libraries/LibWeb/SVG/SVGLineElement.cpp
@@ -66,9 +66,9 @@ GC::Ref<SVGAnimatedLength> SVGLineElement::x1() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_x1.value_or({ 0, false }).value());
-    auto anim_length = SVGLength::create(realm(), 0, m_x1.value_or({ 0, false }).value());
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_x1.value_or({ 0, false }).value(), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_x1.value_or({ 0, false }).value(), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 // https://www.w3.org/TR/SVG11/shapes.html#LineElementY1Attribute
@@ -76,9 +76,9 @@ GC::Ref<SVGAnimatedLength> SVGLineElement::y1() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_y1.value_or({ 0, false }).value());
-    auto anim_length = SVGLength::create(realm(), 0, m_y1.value_or({ 0, false }).value());
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_y1.value_or({ 0, false }).value(), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_y1.value_or({ 0, false }).value(), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 // https://www.w3.org/TR/SVG11/shapes.html#LineElementX2Attribute
@@ -86,9 +86,9 @@ GC::Ref<SVGAnimatedLength> SVGLineElement::x2() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_x2.value_or({ 0, false }).value());
-    auto anim_length = SVGLength::create(realm(), 0, m_x2.value_or({ 0, false }).value());
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_x2.value_or({ 0, false }).value(), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_x2.value_or({ 0, false }).value(), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 // https://www.w3.org/TR/SVG11/shapes.html#LineElementY2Attribute
@@ -96,9 +96,9 @@ GC::Ref<SVGAnimatedLength> SVGLineElement::y2() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_y2.value_or({ 0, false }).value());
-    auto anim_length = SVGLength::create(realm(), 0, m_y2.value_or({ 0, false }).value());
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_y2.value_or({ 0, false }).value(), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_y2.value_or({ 0, false }).value(), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 }
diff --git a/Libraries/LibWeb/SVG/SVGLinearGradientElement.cpp b/Libraries/LibWeb/SVG/SVGLinearGradientElement.cpp
index faa2e5c46c2..29940668c55 100644
--- a/Libraries/LibWeb/SVG/SVGLinearGradientElement.cpp
+++ b/Libraries/LibWeb/SVG/SVGLinearGradientElement.cpp
@@ -162,26 +162,22 @@ Optional<Painting::PaintStyle> SVGLinearGradientElement::to_gfx_paint_style(SVGP
 
 GC::Ref<SVGAnimatedLength> SVGLinearGradientElement::x1() const
 {
-    // FIXME: Implement this properly.
-    return SVGAnimatedLength::create(realm(), SVGLength::create(realm(), 0, 0), SVGLength::create(realm(), 0, 0));
+    return fake_animated_length_fixme();
 }
 
 GC::Ref<SVGAnimatedLength> SVGLinearGradientElement::y1() const
 {
-    // FIXME: Implement this properly.
-    return SVGAnimatedLength::create(realm(), SVGLength::create(realm(), 0, 0), SVGLength::create(realm(), 0, 0));
+    return fake_animated_length_fixme();
 }
 
 GC::Ref<SVGAnimatedLength> SVGLinearGradientElement::x2() const
 {
-    // FIXME: Implement this properly.
-    return SVGAnimatedLength::create(realm(), SVGLength::create(realm(), 0, 0), SVGLength::create(realm(), 0, 0));
+    return fake_animated_length_fixme();
 }
 
 GC::Ref<SVGAnimatedLength> SVGLinearGradientElement::y2() const
 {
-    // FIXME: Implement this properly.
-    return SVGAnimatedLength::create(realm(), SVGLength::create(realm(), 0, 0), SVGLength::create(realm(), 0, 0));
+    return fake_animated_length_fixme();
 }
 
 }
diff --git a/Libraries/LibWeb/SVG/SVGRadialGradientElement.cpp b/Libraries/LibWeb/SVG/SVGRadialGradientElement.cpp
index 412ee052ad3..b8f5fa05c53 100644
--- a/Libraries/LibWeb/SVG/SVGRadialGradientElement.cpp
+++ b/Libraries/LibWeb/SVG/SVGRadialGradientElement.cpp
@@ -218,38 +218,32 @@ Optional<Painting::PaintStyle> SVGRadialGradientElement::to_gfx_paint_style(SVGP
 
 GC::Ref<SVGAnimatedLength> SVGRadialGradientElement::cx() const
 {
-    // FIXME: Implement this properly.
-    return SVGAnimatedLength::create(realm(), SVGLength::create(realm(), 0, 0), SVGLength::create(realm(), 0, 0));
+    return fake_animated_length_fixme();
 }
 
 GC::Ref<SVGAnimatedLength> SVGRadialGradientElement::cy() const
 {
-    // FIXME: Implement this properly.
-    return SVGAnimatedLength::create(realm(), SVGLength::create(realm(), 0, 0), SVGLength::create(realm(), 0, 0));
+    return fake_animated_length_fixme();
 }
 
 GC::Ref<SVGAnimatedLength> SVGRadialGradientElement::fx() const
 {
-    // FIXME: Implement this properly.
-    return SVGAnimatedLength::create(realm(), SVGLength::create(realm(), 0, 0), SVGLength::create(realm(), 0, 0));
+    return fake_animated_length_fixme();
 }
 
 GC::Ref<SVGAnimatedLength> SVGRadialGradientElement::fy() const
 {
-    // FIXME: Implement this properly.
-    return SVGAnimatedLength::create(realm(), SVGLength::create(realm(), 0, 0), SVGLength::create(realm(), 0, 0));
+    return fake_animated_length_fixme();
 }
 
 GC::Ref<SVGAnimatedLength> SVGRadialGradientElement::fr() const
 {
-    // FIXME: Implement this properly.
-    return SVGAnimatedLength::create(realm(), SVGLength::create(realm(), 0, 0), SVGLength::create(realm(), 0, 0));
+    return fake_animated_length_fixme();
 }
 
 GC::Ref<SVGAnimatedLength> SVGRadialGradientElement::r() const
 {
-    // FIXME: Implement this properly.
-    return SVGAnimatedLength::create(realm(), SVGLength::create(realm(), 0, 0), SVGLength::create(realm(), 0, 0));
+    return fake_animated_length_fixme();
 }
 
 }
diff --git a/Libraries/LibWeb/SVG/SVGRectElement.cpp b/Libraries/LibWeb/SVG/SVGRectElement.cpp
index 8f821330c45..1eed1e00eee 100644
--- a/Libraries/LibWeb/SVG/SVGRectElement.cpp
+++ b/Libraries/LibWeb/SVG/SVGRectElement.cpp
@@ -158,9 +158,9 @@ GC::Ref<SVGAnimatedLength> SVGRectElement::x() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_x.value_or(0));
-    auto anim_length = SVGLength::create(realm(), 0, m_x.value_or(0));
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_x.value_or(0), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_x.value_or(0), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 // https://www.w3.org/TR/SVG11/shapes.html#RectElementYAttribute
@@ -168,9 +168,9 @@ GC::Ref<SVGAnimatedLength> SVGRectElement::y() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_y.value_or(0));
-    auto anim_length = SVGLength::create(realm(), 0, m_y.value_or(0));
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_y.value_or(0), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_y.value_or(0), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 // https://www.w3.org/TR/SVG11/shapes.html#RectElementWidthAttribute
@@ -178,9 +178,9 @@ GC::Ref<SVGAnimatedLength> SVGRectElement::width() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_width.value_or(0));
-    auto anim_length = SVGLength::create(realm(), 0, m_width.value_or(0));
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_width.value_or(0), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_width.value_or(0), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 // https://www.w3.org/TR/SVG11/shapes.html#RectElementHeightAttribute
@@ -188,9 +188,9 @@ GC::Ref<SVGAnimatedLength> SVGRectElement::height() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_height.value_or(0));
-    auto anim_length = SVGLength::create(realm(), 0, m_height.value_or(0));
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_height.value_or(0), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_height.value_or(0), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 // https://www.w3.org/TR/SVG11/shapes.html#RectElementRXAttribute
@@ -198,9 +198,9 @@ GC::Ref<SVGAnimatedLength> SVGRectElement::rx() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_radius_x.value_or(0));
-    auto anim_length = SVGLength::create(realm(), 0, m_radius_x.value_or(0));
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_radius_x.value_or(0), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_radius_x.value_or(0), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 // https://www.w3.org/TR/SVG11/shapes.html#RectElementRYAttribute
@@ -208,9 +208,9 @@ GC::Ref<SVGAnimatedLength> SVGRectElement::ry() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_radius_y.value_or(0));
-    auto anim_length = SVGLength::create(realm(), 0, m_radius_y.value_or(0));
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_radius_y.value_or(0), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_radius_y.value_or(0), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 }
diff --git a/Libraries/LibWeb/SVG/SVGSVGElement.cpp b/Libraries/LibWeb/SVG/SVGSVGElement.cpp
index 712d53bfe66..a541e79ea59 100644
--- a/Libraries/LibWeb/SVG/SVGSVGElement.cpp
+++ b/Libraries/LibWeb/SVG/SVGSVGElement.cpp
@@ -274,7 +274,7 @@ void SVGSVGElement::deselect_all() const
 GC::Ref<SVGLength> SVGSVGElement::create_svg_length() const
 {
     // A new, detached SVGLength object whose value is the unitless <number> 0.
-    return SVGLength::create(realm(), SVGLength::SVG_LENGTHTYPE_NUMBER, 0);
+    return SVGLength::create(realm(), SVGLength::SVG_LENGTHTYPE_NUMBER, 0, SVGLength::ReadOnly::No);
 }
 
 GC::Ref<Geometry::DOMPoint> SVGSVGElement::create_svg_point() const
diff --git a/Libraries/LibWeb/SVG/SVGUseElement.cpp b/Libraries/LibWeb/SVG/SVGUseElement.cpp
index 51c579bb164..c5329339a7d 100644
--- a/Libraries/LibWeb/SVG/SVGUseElement.cpp
+++ b/Libraries/LibWeb/SVG/SVGUseElement.cpp
@@ -215,9 +215,9 @@ GC::Ref<SVGAnimatedLength> SVGUseElement::x() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_x.value_or(0));
-    auto anim_length = SVGLength::create(realm(), 0, m_x.value_or(0));
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_x.value_or(0), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_x.value_or(0), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 // https://www.w3.org/TR/SVG11/shapes.html#RectElementYAttribute
@@ -225,21 +225,19 @@ GC::Ref<SVGAnimatedLength> SVGUseElement::y() const
 {
     // FIXME: Populate the unit type when it is parsed (0 here is "unknown").
     // FIXME: Create a proper animated value when animations are supported.
-    auto base_length = SVGLength::create(realm(), 0, m_y.value_or(0));
-    auto anim_length = SVGLength::create(realm(), 0, m_y.value_or(0));
-    return SVGAnimatedLength::create(realm(), move(base_length), move(anim_length));
+    auto base_length = SVGLength::create(realm(), 0, m_y.value_or(0), SVGLength::ReadOnly::No);
+    auto anim_length = SVGLength::create(realm(), 0, m_y.value_or(0), SVGLength::ReadOnly::Yes);
+    return SVGAnimatedLength::create(realm(), base_length, anim_length);
 }
 
 GC::Ref<SVGAnimatedLength> SVGUseElement::width() const
 {
-    // FIXME: Implement this properly.
-    return SVGAnimatedLength::create(realm(), SVGLength::create(realm(), 0, 0), SVGLength::create(realm(), 0, 0));
+    return fake_animated_length_fixme();
 }
 
 GC::Ref<SVGAnimatedLength> SVGUseElement::height() const
 {
-    // FIXME: Implement this properly.
-    return SVGAnimatedLength::create(realm(), SVGLength::create(realm(), 0, 0), SVGLength::create(realm(), 0, 0));
+    return fake_animated_length_fixme();
 }
 
 // https://svgwg.org/svg2-draft/struct.html#TermInstanceRoot
diff --git a/Tests/LibWeb/Text/expected/SVG/svg-rect-animated-length.txt b/Tests/LibWeb/Text/expected/SVG/svg-rect-animated-length.txt
new file mode 100644
index 00000000000..f9df33f76b6
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/SVG/svg-rect-animated-length.txt
@@ -0,0 +1,9 @@
+rect.x.baseVal: 1
+rect.x.animVal: 1
+rect.y.baseVal: 2
+rect.y.animVal: 2
+rect.width.baseVal: 3
+rect.width.animVal: 3
+rect.height.baseVal: 4
+rect.height.animVal: 4
+Expected exception: NoModificationAllowedError: Cannot modify value of read-only SVGLength
diff --git a/Tests/LibWeb/Text/input/SVG/svg-rect-animated-length.html b/Tests/LibWeb/Text/input/SVG/svg-rect-animated-length.html
new file mode 100644
index 00000000000..f6f00bc4c2f
--- /dev/null
+++ b/Tests/LibWeb/Text/input/SVG/svg-rect-animated-length.html
@@ -0,0 +1,23 @@
+<!DOCTYPE html>
+<script src="../include.js"></script>
+<svg xmlns="http://www.w3.org/2000/svg">
+    <rect id="rect" x="1" y="2" width="3" height="4" />
+</svg>
+<script>
+test(() => {
+    println(`rect.x.baseVal: ${rect.x.baseVal.value}`);
+    println(`rect.x.animVal: ${rect.x.animVal.value}`);
+    println(`rect.y.baseVal: ${rect.y.baseVal.value}`);
+    println(`rect.y.animVal: ${rect.y.animVal.value}`);
+    println(`rect.width.baseVal: ${rect.width.baseVal.value}`);
+    println(`rect.width.animVal: ${rect.width.animVal.value}`);
+    println(`rect.height.baseVal: ${rect.height.baseVal.value}`);
+    println(`rect.height.animVal: ${rect.height.animVal.value}`);
+
+    try {
+        rect.x.animVal.value = 2;
+    } catch (e) {
+        println(`Expected exception: ${e}`);
+    }
+});
+</script>