Fix: detect placeholder contrast (#790) (#799)

* Fix: detect placeholder contrast (#790)

`detect` never read `::placeholder` color, so pale placeholders passed. Score them with the same WCAG math as body text, without host class/clip heuristics.

Prepared with AI assistance.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Fix: match descendant ::placeholder hosts (#790)

`.form ::placeholder` kept the ancestor as the host. Reuse the hover combinator star-fill so the color lands on the inputs inside.

Prepared with AI assistance.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Fix: placeholder-shown and gradient alpha (#790)

Browser scans skip when :placeholder-shown is false, so a live filled field does not keep the HTML value attribute's empty state. Translucent placeholders flatten over each gradient stop before scoring.

Prepared with AI assistance.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Fix: trailing combinator only for ::placeholder hosts (#790)

`star_empty_compounds` turned `.label + ::placeholder` into `.label *+*`. Fill only a trailing empty compound so adjacent-sibling hosts still match.

Prepared with AI assistance.

Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/oracle/golden/detect-dir-quiet-all-fixtures.json

@@ -1,6 +1,6 @@
 {
   "stdout": "",
-  "stderr": "415 anti-patterns found.\n17 advisory notes (not counted).\n",
+  "stderr": "419 anti-patterns found.\n17 advisory notes (not counted).\n",
   "exit": 2,
   "signal": null,
   "files": {}

tests/oracle/golden/detect-fixture-json-placeholder-contrast-html.json

@@ -0,0 +1,7 @@
+{
+  "stdout": "[\n  {\n    \"antipattern\": \"low-contrast\",\n    \"name\": \"Low contrast text\",\n    \"description\": \"Text does not meet WCAG AA contrast requirements (4.5:1 for body, 3:1 for large text). Increase the contrast between text and background.\",\n    \"severity\": \"warning\",\n    \"category\": \"quality\",\n    \"file\": \"<REPO>/tests/fixtures/antipatterns/placeholder-contrast.html\",\n    \"line\": 0,\n    \"snippet\": \"placeholder \\\"Pale Placeholder On White Field\\\" 1.9:1 (need 4.5:1) — text #bbbbbb on #ffffff\"\n  },\n  {\n    \"antipattern\": \"low-contrast\",\n    \"name\": \"Low contrast text\",\n    \"description\": \"Text does not meet WCAG AA contrast requirements (4.5:1 for body, 3:1 for large text). Increase the contrast between text and background.\",\n    \"severity\": \"warning\",\n    \"category\": \"quality\",\n    \"file\": \"<REPO>/tests/fixtures/antipatterns/placeholder-contrast.html\",\n    \"line\": 0,\n    \"snippet\": \"placeholder \\\"Pale Placeholder On White Textarea\\\" 1.9:1 (need 4.5:1) — text #bbbbbb on #ffffff\"\n  },\n  {\n    \"antipattern\": \"low-contrast\",\n    \"name\": \"Low contrast text\",\n    \"description\": \"Text does not meet WCAG AA contrast requirements (4.5:1 for body, 3:1 for large text). Increase the contrast between text and background.\",\n    \"severity\": \"warning\",\n    \"category\": \"quality\",\n    \"file\": \"<REPO>/tests/fixtures/antipatterns/placeholder-contrast.html\",\n    \"line\": 0,\n    \"snippet\": \"placeholder \\\"Translucent Placeholder On Light Field\\\" 1.0:1 (need 4.5:1) — text #f9f9f9 on #f5f5f5\"\n  },\n  {\n    \"antipattern\": \"low-contrast\",\n    \"name\": \"Low contrast text\",\n    \"description\": \"Text does not meet WCAG AA contrast requirements (4.5:1 for body, 3:1 for large text). Increase the contrast between text and background.\",\n    \"severity\": \"warning\",\n    \"category\": \"quality\",\n    \"file\": \"<REPO>/tests/fixtures/antipatterns/placeholder-contrast.html\",\n    \"line\": 0,\n    \"snippet\": \"placeholder \\\"Pale Placeholder On Frosted Panel\\\" 3.5:1 (need 4.5:1) — text #bbbbbb on #5c5c5d\"\n  }\n]\n",
+  "stderr": "",
+  "exit": 2,
+  "signal": null,
+  "files": {}
+}

tests/oracle/golden/detect-fixture-text-placeholder-contrast-html.json

@@ -0,0 +1,7 @@
+{
+  "stdout": "",
+  "stderr": "\n<REPO>/tests/fixtures/antipatterns/placeholder-contrast.html\n  [low-contrast] placeholder \"Pale Placeholder On White Field\" 1.9:1 (need 4.5:1) — text #bbbbbb on #ffffff\n    → Text does not meet WCAG AA contrast requirements (4.5:1 for body, 3:1 for large text). Increase the contrast between text and background.\n  [low-contrast] placeholder \"Pale Placeholder On White Textarea\" 1.9:1 (need 4.5:1) — text #bbbbbb on #ffffff\n    → Text does not meet WCAG AA contrast requirements (4.5:1 for body, 3:1 for large text). Increase the contrast between text and background.\n  [low-contrast] placeholder \"Translucent Placeholder On Light Field\" 1.0:1 (need 4.5:1) — text #f9f9f9 on #f5f5f5\n    → Text does not meet WCAG AA contrast requirements (4.5:1 for body, 3:1 for large text). Increase the contrast between text and background.\n  [low-contrast] placeholder \"Pale Placeholder On Frosted Panel\" 3.5:1 (need 4.5:1) — text #bbbbbb on #5c5c5d\n    → Text does not meet WCAG AA contrast requirements (4.5:1 for body, 3:1 for large text). Increase the contrast between text and background.\n\n4 anti-patterns found.\n",
+  "exit": 2,
+  "signal": null,
+  "files": {}
+}