diff --git a/internal/driver/chrome/selector_parity_test.go b/internal/driver/chrome/selector_parity_test.go
new file mode 100644
index 0000000..6d4f1a9
--- /dev/null
+++ b/internal/driver/chrome/selector_parity_test.go
@@ -0,0 +1,124 @@
+//go:build browser
+
+package chrome
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"path/filepath"
+	"slices"
+	"testing"
+	"time"
+
+	"github.com/chromedp/chromedp"
+
+	"github.com/priyanshujain/sanderling/internal/bundler"
+	"github.com/priyanshujain/sanderling/internal/hierarchy"
+)
+
+// One page, two selector matchers.
+//
+// A selector means whatever the runtime resolving it decides. The goja host
+// resolves it against the hierarchy dump through internal/hierarchy; the V8 host
+// resolves it against the live DOM through pkg/spec/src/web-runtime.ts. Nothing
+// makes the two agree, so a spec that targets a list of rows can act on them on
+// one platform and find nothing on the other, and finding nothing is silent: the
+// generator yields no action and the run still passes.
+//
+// This test starts from one real page in a real browser and asks both matchers
+// the same questions.
+func TestSelectors_ResolveTheSameElementsAsTheWebRuntime(t *testing.T) {
+	server := httptest.NewServer(http.FileServer(http.Dir("testdata")))
+	defer server.Close()
+
+	d := New()
+	defer d.Terminate(context.Background())
+	ctx, cancel := context.WithTimeout(context.Background(), 60*time.Second)
+	defer cancel()
+	if err := d.Launch(ctx, server.URL+"/selector-parity.html", false, nil); err != nil {
+		t.Fatalf("Launch: %v", err)
+	}
+
+	dump, err := d.Hierarchy(ctx)
+	if err != nil {
+		t.Fatalf("Hierarchy: %v", err)
+	}
+	tree, err := hierarchy.Parse(dump)
+	if err != nil {
+		t.Fatalf("parse hierarchy: %v", err)
+	}
+	installSelectorProbe(ctx, t, d)
+
+	// want is stated here rather than derived, so a matcher that stops matching
+	// on both sides at once cannot pass this by agreeing on nothing.
+	cases := []struct {
+		selector string
+		want     []string
+	}{
+		{"idPrefix:customer_row_", []string{"customer_row_a1", "customer_row_b2"}},
+		{"idPrefix:customer_", []string{"customer_list", "customer_row_a1", "customer_row_b2"}},
+		{"idPrefix:invoice_row_", nil},
+		{"id:summary_card", []string{"summary_card"}},
+		{"descPrefix:customer_row_", []string{"customer_row_a1", "customer_row_b2"}},
+	}
+	for _, testCase := range cases {
+		t.Run(testCase.selector, func(t *testing.T) {
+			native := selectorIDsFromDump(tree, testCase.selector)
+			web := selectorIDsFromWebRuntime(ctx, t, d, testCase.selector)
+			if !slices.Equal(native, testCase.want) {
+				t.Errorf("hierarchy matched %v, want %v", native, testCase.want)
+			}
+			if !slices.Equal(web, testCase.want) {
+				t.Errorf("the web runtime matched %v, want %v", web, testCase.want)
+			}
+		})
+	}
+}
+
+func selectorIDsFromDump(tree *hierarchy.Tree, selector string) []string {
+	var ids []string
+	for _, element := range tree.FindAll(selector) {
+		ids = append(ids, element.ResourceID)
+	}
+	return ids
+}
+
+func installSelectorProbe(ctx context.Context, t *testing.T, d *Driver) {
+	t.Helper()
+	specSource := filepath.Join(repoRootDir(t), "pkg", "spec")
+	probe, err := bundler.BundleWeb(bundler.WebOptions{
+		EntryFile:      filepath.Join(specSource, "test", "dom-selector-probe.ts"),
+		WebRuntimeFile: filepath.Join(specSource, "src", "web-runtime.ts"),
+	})
+	if err != nil {
+		t.Fatalf("bundle dom selector probe: %v", err)
+	}
+	if err := d.InstallBundle(ctx, probe.JavaScript); err != nil {
+		t.Fatalf("install dom selector probe: %v", err)
+	}
+}
+
+func selectorIDsFromWebRuntime(
+	ctx context.Context,
+	t *testing.T,
+	d *Driver,
+	selector string,
+) []string {
+	t.Helper()
+	request, err := json.Marshal(selector)
+	if err != nil {
+		t.Fatalf("encode selector: %v", err)
+	}
+	var encoded string
+	script := `JSON.stringify(window.__sanderlingSelectorMatches__(` + string(request) + `))`
+	if err := chromedp.Run(d.tabCtx, chromedp.Evaluate(script, &encoded)); err != nil {
+		t.Fatalf("read web runtime matches for %q: %v", selector, err)
+	}
+	var ids []string
+	if err := json.Unmarshal([]byte(encoded), &ids); err != nil {
+		t.Fatalf("decode web runtime matches: %v", err)
+	}
+	return ids
+}
diff --git a/internal/driver/chrome/testdata/selector-parity.html b/internal/driver/chrome/testdata/selector-parity.html
new file mode 100644
index 0000000..2892f46
--- /dev/null
+++ b/internal/driver/chrome/testdata/selector-parity.html
@@ -0,0 +1,15 @@
+<!doctype html>
+<html id="page" lang="en">
+  <head id="page-head">
+    <meta id="page-charset" charset="utf-8" />
+    <title id="page-title">selector parity</title>
+  </head>
+  <body id="page-body">
+    <div id="summary_card" aria-label="summary_card, 3 customers">summary</div>
+    <div id="customer_list" aria-label="customer_list">
+      <div id="customer_row_a1" aria-label="customer_row_a1, Alice">Alice</div>
+      <div id="customer_row_b2" aria-label="customer_row_b2, Bob">Bob</div>
+      <div id="supplier_row_c3" aria-label="supplier_row_c3, Carol">Carol</div>
+    </div>
+  </body>
+</html>
diff --git a/pkg/spec/src/web-runtime.ts b/pkg/spec/src/web-runtime.ts
index 4c38e23..0072926 100644
--- a/pkg/spec/src/web-runtime.ts
+++ b/pkg/spec/src/web-runtime.ts
@@ -589,6 +589,7 @@ installRuntime(
 // so these are stripped from production output; they only exist for unit tests.
 export const __testing__ = {
   host,
+  buildAx,
   seedBigInt,
   collectTargets,
   targetElements,
diff --git a/pkg/spec/test/dom-selector-probe.ts b/pkg/spec/test/dom-selector-probe.ts
new file mode 100644
index 0000000..8d930f6
--- /dev/null
+++ b/pkg/spec/test/dom-selector-probe.ts
@@ -0,0 +1,33 @@
+/// <reference lib="dom" />
+
+// The V8 host's selector matcher, read back match by match.
+//
+// internal/driver/chrome/selector_parity_test.go bundles this probe into a live
+// page and compares what it returns against what internal/hierarchy resolves
+// from the dump of the SAME page. Selector matching is written once per runtime,
+// and only a comparison over one page can say whether the two mean the same
+// thing by one selector. buildAx is the shipped `state.ax`, so the answer comes
+// from production code rather than a copy of it.
+
+import { __testing__ } from "../src/web-runtime.ts";
+
+const { buildAx } = __testing__;
+
+interface Handle {
+  id?: string;
+}
+
+interface Ax {
+  findAll(selector: unknown): Handle[];
+}
+
+function selectorMatches(selector: string): string[] {
+  const ax = buildAx() as Ax;
+  return ax.findAll(selector).map((element) => element.id ?? "");
+}
+
+type SelectorGlobal = {
+  __sanderlingSelectorMatches__: (selector: string) => string[];
+};
+
+(globalThis as unknown as SelectorGlobal).__sanderlingSelectorMatches__ = selectorMatches;