diff --git a/Libraries/LibWeb/DOM/Document.cpp b/Libraries/LibWeb/DOM/Document.cpp
index e4a3eef931d..0e087c0d9a9 100644
--- a/Libraries/LibWeb/DOM/Document.cpp
+++ b/Libraries/LibWeb/DOM/Document.cpp
@@ -1135,10 +1135,10 @@ Vector<CSS::BackgroundLayerData> const* Document::background_layers() const
 
 void Document::update_base_element(Badge<HTML::HTMLBaseElement>)
 {
-    GC::Ptr<HTML::HTMLBaseElement const> base_element_with_href = nullptr;
-    GC::Ptr<HTML::HTMLBaseElement const> base_element_with_target = nullptr;
+    GC::Ptr<HTML::HTMLBaseElement> base_element_with_href = nullptr;
+    GC::Ptr<HTML::HTMLBaseElement> base_element_with_target = nullptr;
 
-    for_each_in_subtree_of_type<HTML::HTMLBaseElement>([&base_element_with_href, &base_element_with_target](HTML::HTMLBaseElement const& base_element_in_tree) {
+    for_each_in_subtree_of_type<HTML::HTMLBaseElement>([&base_element_with_href, &base_element_with_target](HTML::HTMLBaseElement& base_element_in_tree) {
         if (!base_element_with_href && base_element_in_tree.has_attribute(HTML::AttributeNames::href)) {
             base_element_with_href = &base_element_in_tree;
             if (base_element_with_target)
@@ -1157,12 +1157,12 @@ void Document::update_base_element(Badge<HTML::HTMLBaseElement>)
     m_first_base_element_with_target_in_tree_order = base_element_with_target;
 }
 
-GC::Ptr<HTML::HTMLBaseElement const> Document::first_base_element_with_href_in_tree_order() const
+GC::Ptr<HTML::HTMLBaseElement> Document::first_base_element_with_href_in_tree_order() const
 {
     return m_first_base_element_with_href_in_tree_order;
 }
 
-GC::Ptr<HTML::HTMLBaseElement const> Document::first_base_element_with_target_in_tree_order() const
+GC::Ptr<HTML::HTMLBaseElement> Document::first_base_element_with_target_in_tree_order() const
 {
     return m_first_base_element_with_target_in_tree_order;
 }
diff --git a/Libraries/LibWeb/DOM/Document.h b/Libraries/LibWeb/DOM/Document.h
index cc68e4d97af..22edbdb4610 100644
--- a/Libraries/LibWeb/DOM/Document.h
+++ b/Libraries/LibWeb/DOM/Document.h
@@ -223,8 +223,8 @@ public:
     URL::URL base_url() const;
 
     void update_base_element(Badge<HTML::HTMLBaseElement>);
-    GC::Ptr<HTML::HTMLBaseElement const> first_base_element_with_href_in_tree_order() const;
-    GC::Ptr<HTML::HTMLBaseElement const> first_base_element_with_target_in_tree_order() const;
+    GC::Ptr<HTML::HTMLBaseElement> first_base_element_with_href_in_tree_order() const;
+    GC::Ptr<HTML::HTMLBaseElement> first_base_element_with_target_in_tree_order() const;
 
     String url_string() const { return m_url.to_string(); }
     String document_uri() const { return url_string(); }
@@ -1135,8 +1135,8 @@ private:
     GC::Ptr<Selection::Selection> m_selection;
 
     // NOTE: This is a cache to make finding the first <base href> or <base target> element O(1).
-    GC::Ptr<HTML::HTMLBaseElement const> m_first_base_element_with_href_in_tree_order;
-    GC::Ptr<HTML::HTMLBaseElement const> m_first_base_element_with_target_in_tree_order;
+    GC::Ptr<HTML::HTMLBaseElement> m_first_base_element_with_href_in_tree_order;
+    GC::Ptr<HTML::HTMLBaseElement> m_first_base_element_with_target_in_tree_order;
 
     // https://html.spec.whatwg.org/multipage/images.html#list-of-available-images
     GC::Ptr<HTML::ListOfAvailableImages> m_list_of_available_images;
diff --git a/Libraries/LibWeb/HTML/HTMLBaseElement.cpp b/Libraries/LibWeb/HTML/HTMLBaseElement.cpp
index 4cba0578e1e..bacda241da2 100644
--- a/Libraries/LibWeb/HTML/HTMLBaseElement.cpp
+++ b/Libraries/LibWeb/HTML/HTMLBaseElement.cpp
@@ -43,7 +43,14 @@ void HTMLBaseElement::inserted()
 void HTMLBaseElement::removed_from(Node* old_parent, Node& old_root)
 {
     HTMLElement::removed_from(old_parent, old_root);
+    auto old_first_base_element_with_href_in_tree_order = document().first_base_element_with_href_in_tree_order();
     document().update_base_element({});
+
+    // The frozen base URL must be immediately set for an element whenever any of the following situations occur:
+    // - The base element becomes the first base element in tree order with an href content attribute in its Document.
+    auto first_base_element_with_href_in_document = document().first_base_element_with_href_in_tree_order();
+    if (first_base_element_with_href_in_document != old_first_base_element_with_href_in_tree_order)
+        first_base_element_with_href_in_document->set_the_frozen_base_url();
 }
 
 void HTMLBaseElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value, Optional<FlyString> const& namespace_)
@@ -72,13 +79,17 @@ void HTMLBaseElement::set_the_frozen_base_url()
     auto href = get_attribute_value(AttributeNames::href);
     auto url_record = document.fallback_base_url().complete_url(href);
 
-    // 3. Set element's frozen base URL to document's fallback base URL, if urlRecord is failure or running Is base allowed for Document? on the resulting URL record and document returns "Blocked", and to urlRecord otherwise.
-    // FIXME: Apply "Is base allowed for Document?" CSP
-    if (!url_record.has_value()) {
+    // 3. If any of the following are true:
+    // - urlRecord is failure;
+    // - urlRecord's scheme is "data" or "javascript"; or
+    // FIXME: - running Is base allowed for Document? on urlRecord and document returns "Blocked",
+    // then set element's frozen base URL to document's fallback base URL and return.
+    if (!url_record.has_value() || url_record->scheme() == "data" || url_record->scheme() == "javascript") {
         m_frozen_base_url = document.fallback_base_url();
         return;
     }
 
+    // 4. Set element's frozen base URL to urlRecord.
     m_frozen_base_url = url_record.release_value();
 }
 
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/document-metadata/the-base-element/base-data.txt b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/document-metadata/the-base-element/base-data.txt
new file mode 100644
index 00000000000..fdd6341c3a3
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/document-metadata/the-base-element/base-data.txt
@@ -0,0 +1,8 @@
+Harness status: OK
+
+Found 3 tests
+
+3 Pass
+Pass	First <base> has a data: URL so fallback is used
+Pass	First <base> is removed so second is used
+Pass	Dynamically inserted first <base> has a data: URL so fallback is used
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/document-metadata/the-base-element/base-javascript.txt b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/document-metadata/the-base-element/base-javascript.txt
new file mode 100644
index 00000000000..a32e8f7bf7b
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/document-metadata/the-base-element/base-javascript.txt
@@ -0,0 +1,8 @@
+Harness status: OK
+
+Found 3 tests
+
+3 Pass
+Pass	First <base> has a javascript: URL so fallback is used
+Pass	First <base> is removed so second is used
+Pass	Dynamically inserted first <base> has a javascript: URL so fallback is used
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/semantics/document-metadata/the-base-element/base-data.html b/Tests/LibWeb/Text/input/wpt-import/html/semantics/document-metadata/the-base-element/base-data.html
new file mode 100644
index 00000000000..9109d4b6787
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/semantics/document-metadata/the-base-element/base-data.html
@@ -0,0 +1,32 @@
+<!-- Please update base-javascript.html together with this -->
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>&lt;base> and data: URLs</title>
+<script src=../../../../resources/testharness.js></script>
+<script src=../../../../resources/testharnessreport.js></script>
+<base href="data:/,test">
+<base href="https://example.com/">
+<div id=log></div>
+<script>
+test(() => {
+  const link = document.createElement("a");
+  link.href = "blah";
+  assert_equals(link.href, new URL("blah", document.URL).href);
+}, "First <base> has a data: URL so fallback is used");
+
+test(() => {
+  document.querySelector("base").remove();
+  const link = document.createElement("a");
+  link.href = "blah";
+  assert_equals(link.href, new URL("blah", "https://example.com/").href);
+}, "First <base> is removed so second is used");
+
+test(() => {
+  const base = document.createElement("base");
+  base.href = "data:/,more-test";
+  document.head.prepend(base);
+  const link = document.createElement("a");
+  link.href = "blah";
+  assert_equals(link.href, new URL("blah", document.URL).href);
+}, "Dynamically inserted first <base> has a data: URL so fallback is used");
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/semantics/document-metadata/the-base-element/base-javascript.html b/Tests/LibWeb/Text/input/wpt-import/html/semantics/document-metadata/the-base-element/base-javascript.html
new file mode 100644
index 00000000000..0cc72165705
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/semantics/document-metadata/the-base-element/base-javascript.html
@@ -0,0 +1,32 @@
+<!-- Please update base-data.html together with this -->
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>&lt;base> and javascript: URLs</title>
+<script src=../../../../resources/testharness.js></script>
+<script src=../../../../resources/testharnessreport.js></script>
+<base href="javascript:/,test">
+<base href="https://example.com/">
+<div id=log></div>
+<script>
+test(() => {
+  const link = document.createElement("a");
+  link.href = "blah";
+  assert_equals(link.href, new URL("blah", document.URL).href);
+}, "First <base> has a javascript: URL so fallback is used");
+
+test(() => {
+  document.querySelector("base").remove();
+  const link = document.createElement("a");
+  link.href = "blah";
+  assert_equals(link.href, new URL("blah", "https://example.com/").href);
+}, "First <base> is removed so second is used");
+
+test(() => {
+  const base = document.createElement("base");
+  base.href = "javascript:/,more-test";
+  document.head.prepend(base);
+  const link = document.createElement("a");
+  link.href = "blah";
+  assert_equals(link.href, new URL("blah", document.URL).href);
+}, "Dynamically inserted first <base> has a javascript: URL so fallback is used");
+</script>