diff --git a/.markdownlint.jsonc b/.markdownlint.jsonc
index 5bb339c1..d2819b43 100644
--- a/.markdownlint.jsonc
+++ b/.markdownlint.jsonc
@@ -3,78 +3,16 @@
 // - severity: "warning": reported but doesn't fail (non-blocking)
 // - severity: "error": fails validation (default, if not specified)
 
-// WARNINGS: Non-blocking formatting suggestions (for compatibility with existing content)
+// OVERRIDES: project-specific adjustments (all other enabled markdownlint rules use default severity: "error")
+// Rationale / hardening plan: docs/adr/009-markdown-validation.md
 {
-  "blanks-around-fences": {
-    "severity": "warning"
-  },
-  "blanks-around-headings": {
-    "severity": "warning"
-  },
-  "blanks-around-lists": {
-    "severity": "warning"
-  },
-  "blanks-around-tables": {
-    "severity": "warning"
-  },
-  "code-block-style": {
-    "severity": "warning"
-  },
   "line-length": {
-    "severity": "warning",
+    "severity": "error",
     "line_length": 120,
     "tables": false
   },
-  "list-indent": {
-    "severity": "warning"
-  },
-  "fenced-code-language": {
-    "severity": "warning"
-  },
-  "first-line-heading": {
-    "severity": "warning"
-  },
-  "heading-style": {
-    "severity": "warning"
-  },
-  "no-bare-urls": {
-    "severity": "warning"
-  },
-  "no-duplicate-heading": {
-    "severity": "warning"
-  },
-  "no-emphasis-as-heading": {
-    "severity": "warning"
-  },
   "no-inline-html": {
     // Allowed elements for inline HTML in content
     "allowed_elements": ["a", "img"]
-  },
-  "no-multiple-blanks": {
-    "severity": "warning"
-  },
-  "no-multiple-space-atx": {
-    "severity": "warning"
-  },
-  "no-trailing-punctuation": {
-    "severity": "warning"
-  },
-  "no-trailing-spaces": {
-    "severity": "warning"
-  },
-  "ol-prefix": {
-    "severity": "warning"
-  },
-  "single-title": {
-    "severity": "warning"
-  },
-  "table-column-style": {
-    "severity": "warning"
-  },
-  "ul-indent": {
-    "severity": "warning"
-  },
-  "ul-style": {
-    "severity": "warning"
   }
 }