diff --git a/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp b/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp
index 039baef4041..e4e83055073 100644
--- a/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp
+++ b/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp
@@ -199,6 +199,9 @@ void HTMLParser::run(HTMLTokenizer::StopAtInsertionPoint stop_at_insertion_point
dbgln_if(HTML_PARSER_DEBUG, "[{}] {}", insertion_mode_name(), token.to_string());
+ if (token.is_end_of_file() && m_tokenizer.is_eof_inserted())
+ break;
+
// https://html.spec.whatwg.org/multipage/parsing.html#tree-construction-dispatcher
// As each token is emitted from the tokenizer, the user agent must follow the appropriate steps from the following list, known as the tree construction dispatcher:
if (m_stack_of_open_elements.is_empty()
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/quirks.window.txt b/Tests/LibWeb/Text/expected/wpt-import/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/quirks.window.txt
new file mode 100644
index 00000000000..4cc431a0fa0
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/quirks.window.txt
@@ -0,0 +1,10 @@
+Harness status: OK
+
+Found 4 tests
+
+3 Pass
+1 Fail
+Fail document.open() sets document to no-quirks mode (write no doctype)
+Pass document.open() sets document to no-quirks mode (write old doctype)
+Pass document.open() sets document to no-quirks mode (write new doctype)
+Pass document.open() sets document to no-quirks mode, not limited-quirks mode
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/quirks.window.html b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/quirks.window.html
new file mode 100644
index 00000000000..d97a48718af
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/quirks.window.html
@@ -0,0 +1,8 @@
+
+
+
+
+
+
+
+
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/quirks.window.js b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/quirks.window.js
new file mode 100644
index 00000000000..0ff0bb99443
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/quirks.window.js
@@ -0,0 +1,74 @@
+test(t => {
+ const frame = document.body.appendChild(document.createElement("iframe"));
+ t.add_cleanup(() => frame.contentDocument.close());
+ assert_equals(frame.contentDocument.compatMode, "BackCompat");
+ frame.contentDocument.open();
+ assert_equals(frame.contentDocument.compatMode, "CSS1Compat");
+ frame.contentDocument.close();
+ assert_equals(frame.contentDocument.compatMode, "BackCompat");
+}, "document.open() sets document to no-quirks mode (write no doctype)");
+
+test(t => {
+ const frame = document.body.appendChild(document.createElement("iframe"));
+ t.add_cleanup(() => frame.contentDocument.close());
+ assert_equals(frame.contentDocument.compatMode, "BackCompat");
+ frame.contentDocument.open();
+ assert_equals(frame.contentDocument.compatMode, "CSS1Compat");
+ frame.contentDocument.write("");
+ assert_equals(frame.contentDocument.compatMode, "BackCompat");
+ frame.contentDocument.close();
+ assert_equals(frame.contentDocument.compatMode, "BackCompat");
+}, "document.open() sets document to no-quirks mode (write old doctype)");
+
+test(t => {
+ const frame = document.body.appendChild(document.createElement("iframe"));
+ t.add_cleanup(() => frame.contentDocument.close());
+ assert_equals(frame.contentDocument.compatMode, "BackCompat");
+ frame.contentDocument.open();
+ assert_equals(frame.contentDocument.compatMode, "CSS1Compat");
+ frame.contentDocument.write("");
+ assert_equals(frame.contentDocument.compatMode, "CSS1Compat");
+ frame.contentDocument.close();
+ assert_equals(frame.contentDocument.compatMode, "CSS1Compat");
+}, "document.open() sets document to no-quirks mode (write new doctype)");
+
+// This tests the document.open() call in fact sets the document to no-quirks
+// mode, not limited-quirks mode. It is derived from
+// quirks/blocks-ignore-line-height.html in WPT, as there is no direct way to
+// distinguish between a no-quirks document and a limited-quirks document. It
+// assumes that the user agent passes the linked test, which at the time of
+// writing is all major web browsers.
+test(t => {
+ const frame = document.body.appendChild(document.createElement("iframe"));
+ t.add_cleanup(() => frame.contentDocument.close());
+ assert_equals(frame.contentDocument.compatMode, "BackCompat");
+ frame.contentDocument.open();
+ assert_equals(frame.contentDocument.compatMode, "CSS1Compat");
+
+ // Create the DOM tree manually rather than going through document.write() to
+ // bypass the parser, which resets the document mode.
+ const html = frame.contentDocument.appendChild(frame.contentDocument.createElement("html"));
+ const body = html.appendChild(frame.contentDocument.createElement("body"));
+ assert_equals(frame.contentDocument.body, body);
+ body.innerHTML = `
+
+ x
+ x
+ x
+ `;
+ assert_equals(frame.contentDocument.compatMode, "CSS1Compat");
+
+ const idTest = frame.contentDocument.getElementById("test");
+ const idRef = frame.contentDocument.getElementById("ref");
+ const idSRef = frame.contentDocument.getElementById("s_ref");
+ assert_equals(frame.contentWindow.getComputedStyle(idTest).height,
+ frame.contentWindow.getComputedStyle(idSRef).height);
+ assert_not_equals(frame.contentWindow.getComputedStyle(idTest).height,
+ frame.contentWindow.getComputedStyle(idRef).height);
+}, "document.open() sets document to no-quirks mode, not limited-quirks mode");