diff --git a/.Jules/palette.md b/.Jules/palette.md
index d8f0c25d5..7bd5fb8a5 100644
--- a/.Jules/palette.md
+++ b/.Jules/palette.md
@@ -23,3 +23,6 @@
 ## 2026-08-04 - Status Semantics and Numeric Alignment for Reports
 **Learning:** Explicit status semantics can make conditionally rendered empty states easier to discover with assistive technology, while tabular numerals improve visual comparison of metric columns. Focus-reveal behavior must not depend only on `:focus-visible`, and hover styling must not reduce the contrast of unrelated rows.
 **Action:** Use `role="status"` for genuine conditionally rendered status messages, apply `font-variant-numeric: tabular-nums` to numeric report tables, reveal skip links on `:focus`, retain a visible `:focus-visible` indicator, and avoid opacity-based dimming of non-hovered content.
+## 2025-05-18 - Prevent Mouse Focus Rings on Summary Elements
+**Learning:** Native `<summary>` elements receive focus when clicked, causing browsers to display default focus rings that can be visually jarring for mouse users.
+**Action:** Apply `outline: none` to the `:focus` state of `<summary>` elements while retaining custom, high-visibility outlines on their `:focus-visible` state to ensure both mouse and keyboard users receive appropriate feedback.
diff --git a/python/fast_mlsirm/report.py b/python/fast_mlsirm/report.py
index b8627f83d..f988e1434 100644
--- a/python/fast_mlsirm/report.py
+++ b/python/fast_mlsirm/report.py
@@ -217,9 +217,7 @@ def _render_dimensionality_report(payload: dict[str, Any]) -> list[str]:
 def _metric_section(heading: str, metrics: dict[str, Any]) -> str | None:
     """Render a labelled metric-card grid, or ``None`` when there are no metrics."""
     if len(metrics) > MAX_REPORT_METRICS:
-        raise ValueError(
-            f"report metrics exceed the {MAX_REPORT_METRICS}-entry limit"
-        )
+        raise ValueError(f"report metrics exceed the {MAX_REPORT_METRICS}-entry limit")
 
     cards = []
     for key, value in metrics.items():
@@ -443,13 +441,9 @@ def _rows_from_columnar(section: Any) -> list[dict[str, Any]]:
     if row_count == 0:
         return []
     if row_count > MAX_REPORT_ROWS:
-        raise ValueError(
-            f"report table rows exceed the {MAX_REPORT_ROWS}-row limit"
-        )
+        raise ValueError(f"report table rows exceed the {MAX_REPORT_ROWS}-row limit")
     if row_count * len(columns) > MAX_REPORT_CELLS:
-        raise ValueError(
-            f"report table cells exceed the {MAX_REPORT_CELLS}-cell limit"
-        )
+        raise ValueError(f"report table cells exceed the {MAX_REPORT_CELLS}-cell limit")
 
     rows = []
     for index in range(row_count):
@@ -463,9 +457,7 @@ def _rows_from_columnar(section: Any) -> list[dict[str, Any]]:
 def _validate_report_rows(rows: list[dict[str, Any]]) -> None:
     """Reject row-oriented diagnostics that exceed report rendering budgets."""
     if len(rows) > MAX_REPORT_ROWS:
-        raise ValueError(
-            f"report table rows exceed the {MAX_REPORT_ROWS}-row limit"
-        )
+        raise ValueError(f"report table rows exceed the {MAX_REPORT_ROWS}-row limit")
 
     columns: set[str] = set()
     cell_count = 0
@@ -925,6 +917,11 @@ def _css() -> str:
   color: var(--teal);
 }
 
+.exact-values > summary:focus,
+.export-block > summary:focus {
+  outline: none;
+}
+
 .exact-values > summary:focus-visible,
 .export-block > summary:focus-visible {
   outline: 3px solid var(--teal);