diff --git a/justfile b/justfile
index 604cd44e..8922a45f 100644
--- a/justfile
+++ b/justfile
@@ -72,8 +72,8 @@ wpt-test filter="": wpt-build
 
 # Update web platform test expectations
 [group('wpt')]
-wpt-update: wpt-build
-    WPT_FLAGS="--update-expectations" ctest --test-dir {{ builddir }} -R wpt --verbose
+wpt-update filter="": wpt-build
+    WPT_FLAGS="--update-expectations" WPT_FILTER={{ filter }} ctest --test-dir {{ builddir }} -R wpt --verbose
 
 # Run wpt server
 [group('wpt')]
diff --git a/tests/wpt-harness/expectations/compression/compression-large-flush-output.any.js.json b/tests/wpt-harness/expectations/compression/compression-large-flush-output.any.js.json
index 32813d9d..8a19d371 100644
--- a/tests/wpt-harness/expectations/compression/compression-large-flush-output.any.js.json
+++ b/tests/wpt-harness/expectations/compression/compression-large-flush-output.any.js.json
@@ -1,11 +1,11 @@
 {
   "deflate compression with large flush output": {
-    "status": "FAIL"
+    "status": "PASS"
   },
   "gzip compression with large flush output": {
-    "status": "FAIL"
+    "status": "PASS"
   },
   "deflate-raw compression with large flush output": {
-    "status": "FAIL"
+    "status": "PASS"
   }
 }
\ No newline at end of file
diff --git a/tests/wpt-harness/expectations/compression/compression-with-detach.tentative.window.js.json b/tests/wpt-harness/expectations/compression/compression-with-detach.tentative.window.js.json
index 602b2f85..bc5edcde 100644
--- a/tests/wpt-harness/expectations/compression/compression-with-detach.tentative.window.js.json
+++ b/tests/wpt-harness/expectations/compression/compression-with-detach.tentative.window.js.json
@@ -1,5 +1,5 @@
 {
   "data should be correctly compressed even if input is detached partway": {
-    "status": "FAIL"
+    "status": "PASS"
   }
 }
\ No newline at end of file
diff --git a/tests/wpt-harness/expectations/compression/decompression-with-detach.tentative.window.js.json b/tests/wpt-harness/expectations/compression/decompression-with-detach.tentative.window.js.json
index 56e75f4d..5fb147e3 100644
--- a/tests/wpt-harness/expectations/compression/decompression-with-detach.tentative.window.js.json
+++ b/tests/wpt-harness/expectations/compression/decompression-with-detach.tentative.window.js.json
@@ -1,5 +1,5 @@
 {
   "data should be correctly decompressed even if input is detached partway": {
-    "status": "FAIL"
+    "status": "PASS"
   }
 }
\ No newline at end of file
diff --git a/tests/wpt-harness/post-harness.js b/tests/wpt-harness/post-harness.js
index be9d6175..591f5725 100644
--- a/tests/wpt-harness/post-harness.js
+++ b/tests/wpt-harness/post-harness.js
@@ -13,8 +13,8 @@ async function handleRequest(event) {
   let url = new URL(event.request.url);
   console.log(`running test ${url.pathname}`);
   let input = `http://web-platform.test:8000${url.pathname}${url.search}`;
-  globalThis.baseURL = new URL(input);
-  globalThis.location = baseURL;
+  globalThis.wpt_baseURL = new URL(input);
+  globalThis.location = wpt_baseURL;
   try {
     let response = await fetch(input);
     if (!response.ok) {
@@ -52,32 +52,13 @@ async function handleRequest(event) {
 
 function evalAllScripts(wpt_test_scripts) {
   for (let wpt_test_script of wpt_test_scripts) {
-    eval(wpt_test_script);
+    evalScript(wpt_test_script);
   }
 }
 
 async function loadMetaScript(path) {
   let response = await fetch(path);
   let metaSource = await response.text();
-  // Somewhat annoyingly, the WPT harness includes META scripts as <script src=[path]> tags,
-  // which don't create their own scope for `const` and `let bindings, as `eval` does.
-  // That means that running them in `eval` doesn't make any `const` and `let` bound values
-  // available to code outside of the current `eval`, which breaks various WPT files.
-  //
-  // Short of introducing a host call for evaluating code in the same way a `<script>` tag does
-  // the only way to get around that is to replace all these bindings with `var`. Which is
-  // an ugly hack, but works reasonably well.
-  let lines = metaSource.split("\n");
-  lines = lines.map(line => {
-    if (line.indexOf("const ") == 0) {
-      return "var " + line.substr(6);
-    }
-    if (line.indexOf("let ") == 0) {
-      return "var " + line.substr(4);
-    }
-    return line;
-  });
-  metaSource = lines.join("\n");
   metaSource += "\n//# sourceURL=" + path;
   return metaSource;
 }
diff --git a/tests/wpt-harness/wpt_builtins.cpp b/tests/wpt-harness/wpt_builtins.cpp
index e5b7eb4b..4430f996 100644
--- a/tests/wpt-harness/wpt_builtins.cpp
+++ b/tests/wpt-harness/wpt_builtins.cpp
@@ -1,5 +1,8 @@
 #include "extension-api.h"
 
+#include <encode.h>
+#include <js/CompilationAndEvaluation.h>
+#include <js/SourceText.h>
 #include <url.h>
 #include <worker-location.h>
 
@@ -26,14 +29,36 @@ static bool baseURL_get(JSContext *cx, unsigned argc, JS::Value *vp) {
   return true;
 }
 
+/**
+ * Evaluate the given script in the global scope, without creating a new lexical scope.
+ * This is roughly equivalent to how `<script>` tags work in HTML, and hence how
+ * the WPT harness needs to load `META` scripts: otherwise, `let` and `const` bindings
+ * aren't visible to importing code, and the tests break.
+ */
+static bool evalScript(JSContext *cx, unsigned argc, JS::Value *vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  auto script = core::encode(cx, args.get(0));
+  if (!script) {
+    return false;
+  }
+  JS::SourceText<mozilla::Utf8Unit> source;
+  MOZ_RELEASE_ASSERT(source.init(cx, std::move(script.ptr), script.len));
+  JS::CompileOptions options(cx);
+  options.setNonSyntacticScope(true);
+  return Evaluate(cx, options, source, args.rval());
+}
+
 
 const JSPropertySpec properties[] = {
-  JS_PSGS("baseURL", baseURL_get, baseURL_set, JSPROP_ENUMERATE),
+  JS_PSGS("wpt_baseURL", baseURL_get, baseURL_set, JSPROP_ENUMERATE),
 JS_PS_END};
 
 namespace wpt_support {
 
 bool install(api::Engine* engine) {
+  if (!JS_DefineFunction(engine->cx(), engine->global(), "evalScript", evalScript, 1, 0)) {
+    return false;
+  }
   if (!JS_DefineProperties(engine->cx(), engine->global(), properties)) {
     return false;
   }