Add [hidden][hidden] { display: none !important}

/cc @jonrohan — I did this here rather than in normalize.scss because I didn't want to lose it if we ever update that file again.

modules/primer-base/lib/base.scss

@@ -68,6 +68,12 @@ button {
   border-radius: 0;
 }
 
+// increase the selector specificity for [hidden]
+// so that it always overrides utility classes (.d-block, etc.)
+[hidden][hidden] {
+  display: none !important;
+}
+
 details {
   summary { cursor: pointer; }