diff --git a/.github/workflows/linux-sdk-contracts.yml b/.github/workflows/linux-sdk-contracts.yml
index a1cfe3616..c0fcff177 100644
--- a/.github/workflows/linux-sdk-contracts.yml
+++ b/.github/workflows/linux-sdk-contracts.yml
@@ -15,6 +15,7 @@ jobs:
       - name: Header scanner regressions and public dependency closure
         run: |
           python3 eng/sdk/test-header-closure.py
+          python3 eng/sdk/test-verify-parser-components.py
           python3 eng/sdk/test-qualify-macos-profile.py
           python3 - <<'PY'
           import importlib.util
diff --git a/docs/validation/parser-component-size-budget-671-229.md b/docs/validation/parser-component-size-budget-671-229.md
new file mode 100644
index 000000000..60e2cdf6d
--- /dev/null
+++ b/docs/validation/parser-component-size-budget-671-229.md
@@ -0,0 +1,24 @@
+# Parser component size budget (#671 / #229)
+
+The CSS/selector parser archive is a reusable SDK component. It must remain
+free of the runtime HTML parser while CSS and selector coverage grows.
+
+`eng/sdk/verify-parser-components.py` enforces three independent boundaries:
+
+- required CSS and selector ABI symbols must be present;
+- HTML ABI symbols and `html5ever::` symbols must be absent;
+- the focused archive must be no more than 90% of the full parser archive and
+  no more than 18,000,000 bytes.
+
+The ratio detects a collapsed component split. The absolute limit detects
+package growth even when the full parser grows at the same rate. Symbol checks
+remain the semantic proof that HTML code was not retained.
+
+The Linux observation that triggered this update was 16,511,566 bytes for the
+CSS/selector archive and 19,346,060 bytes for the full archive, a ratio of
+0.853485. Both byte values and both configured limits are emitted in
+`parser-components.json` for later release/package-size comparison.
+
+`eng/sdk/test-verify-parser-components.py` locks the passing boundaries and
+independent failures for ratio, bytes, missing CSS ABI, retained HTML ABI, and
+retained `html5ever` symbols.
diff --git a/eng/sdk/test-verify-parser-components.py b/eng/sdk/test-verify-parser-components.py
new file mode 100644
index 000000000..2fa8ff5c1
--- /dev/null
+++ b/eng/sdk/test-verify-parser-components.py
@@ -0,0 +1,54 @@
+#!/usr/bin/env python3
+import importlib.util
+from pathlib import Path
+import unittest
+
+
+SCRIPT = Path(__file__).with_name("verify-parser-components.py")
+SPEC = importlib.util.spec_from_file_location("verify_parser_components", SCRIPT)
+MODULE = importlib.util.module_from_spec(SPEC)
+SPEC.loader.exec_module(MODULE)
+
+
+class VerifyParserComponentsTests(unittest.TestCase):
+    @staticmethod
+    def full_symbols():
+        return "\n".join(MODULE.HTML_SYMBOLS + MODULE.CSS_SYMBOLS)
+
+    @staticmethod
+    def css_symbols():
+        return "\n".join(MODULE.CSS_SYMBOLS)
+
+    def test_accepts_both_budget_boundaries(self):
+        report = MODULE.verify_components(
+            self.full_symbols(), self.css_symbols(), 20_000_000, 18_000_000)
+        self.assertEqual(report["maximumCssToFullSizeRatio"], 0.90)
+        self.assertEqual(report["maximumCssSelectorArchiveBytes"], 18_000_000)
+
+    def test_ratio_budget_fails_independently(self):
+        with self.assertRaisesRegex(RuntimeError, "size ratio"):
+            MODULE.verify_components(
+                self.full_symbols(), self.css_symbols(), 10_000_000, 9_000_001)
+
+    def test_absolute_budget_fails_independently(self):
+        with self.assertRaisesRegex(RuntimeError, "archive size"):
+            MODULE.verify_components(
+                self.full_symbols(), self.css_symbols(), 25_000_000, 18_000_001)
+
+    def test_missing_css_abi_fails(self):
+        with self.assertRaisesRegex(RuntimeError, "missingCss"):
+            MODULE.verify_components(
+                self.full_symbols(), self.css_symbols().replace(MODULE.CSS_SYMBOLS[0], ""),
+                20_000_000, 10_000_000)
+
+    def test_retained_html_symbols_fail(self):
+        for retained in (MODULE.HTML_SYMBOLS[0], "html5ever::"):
+            with self.subTest(retained=retained), self.assertRaisesRegex(
+                    RuntimeError, "retainedHtml"):
+                MODULE.verify_components(
+                    self.full_symbols(), self.css_symbols() + "\n" + retained,
+                    20_000_000, 10_000_000)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/eng/sdk/verify-parser-components.py b/eng/sdk/verify-parser-components.py
index e6269cc69..86b79faaf 100755
--- a/eng/sdk/verify-parser-components.py
+++ b/eng/sdk/verify-parser-components.py
@@ -17,6 +17,8 @@
     "webscene_selector_parser_abi_version",
     "webscene_selector_parse",
 )
+MAXIMUM_CSS_TO_FULL_SIZE_RATIO = 0.90
+MAXIMUM_CSS_SELECTOR_ARCHIVE_BYTES = 18_000_000
 
 
 def symbols(nm, archive):
@@ -24,18 +26,7 @@ def symbols(nm, archive):
         [str(nm), "-g", "-C", str(archive)], text=True, stderr=subprocess.STDOUT)
 
 
-def main():
-    parser = argparse.ArgumentParser(description=__doc__)
-    parser.add_argument("--nm", required=True, type=Path)
-    parser.add_argument("--full", required=True, type=Path)
-    parser.add_argument("--css", required=True, type=Path)
-    parser.add_argument("--output", required=True, type=Path)
-    args = parser.parse_args()
-    for path in (args.nm, args.full, args.css):
-        if not path.is_file():
-            parser.error(f"missing input: {path}")
-    full = symbols(args.nm, args.full)
-    css = symbols(args.nm, args.css)
+def verify_components(full, css, full_size, css_size):
     missing_full = [name for name in HTML_SYMBOLS + CSS_SYMBOLS if name not in full]
     missing_css = [name for name in CSS_SYMBOLS if name not in css]
     retained_html = [name for name in HTML_SYMBOLS if name in css]
@@ -47,20 +38,40 @@ def main():
             "missingCss": missing_css,
             "retainedHtml": retained_html,
         }, sort_keys=True))
-    full_size = args.full.stat().st_size
-    css_size = args.css.stat().st_size
     ratio = css_size / full_size
-    if ratio > 0.85:
+    if ratio > MAXIMUM_CSS_TO_FULL_SIZE_RATIO:
         raise RuntimeError(
-            f"CSS/selector archive size ratio {ratio:.6f} exceeds 0.85 "
-            f"({css_size}/{full_size})")
-    report = {
+            f"CSS/selector archive size ratio {ratio:.6f} exceeds "
+            f"{MAXIMUM_CSS_TO_FULL_SIZE_RATIO:.2f} ({css_size}/{full_size})")
+    if css_size > MAXIMUM_CSS_SELECTOR_ARCHIVE_BYTES:
+        raise RuntimeError(
+            f"CSS/selector archive size {css_size} exceeds "
+            f"{MAXIMUM_CSS_SELECTOR_ARCHIVE_BYTES} bytes")
+    return {
         "schemaVersion": 1,
         "fullArchive": {"bytes": full_size, "html": True, "css": True, "selectors": True},
         "cssSelectorArchive": {"bytes": css_size, "html": False, "css": True, "selectors": True},
         "cssToFullSizeRatio": ratio,
-        "maximumCssToFullSizeRatio": 0.85,
+        "maximumCssToFullSizeRatio": MAXIMUM_CSS_TO_FULL_SIZE_RATIO,
+        "maximumCssSelectorArchiveBytes": MAXIMUM_CSS_SELECTOR_ARCHIVE_BYTES,
     }
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--nm", required=True, type=Path)
+    parser.add_argument("--full", required=True, type=Path)
+    parser.add_argument("--css", required=True, type=Path)
+    parser.add_argument("--output", required=True, type=Path)
+    args = parser.parse_args()
+    for path in (args.nm, args.full, args.css):
+        if not path.is_file():
+            parser.error(f"missing input: {path}")
+    full = symbols(args.nm, args.full)
+    css = symbols(args.nm, args.css)
+    full_size = args.full.stat().st_size
+    css_size = args.css.stat().st_size
+    report = verify_components(full, css, full_size, css_size)
     args.output.parent.mkdir(parents=True, exist_ok=True)
     args.output.write_text(json.dumps(report, indent=2) + "\n")
     print(json.dumps(report, sort_keys=True))