Move annotation of field validation failures to CSS

netbox/project-static/styles/overrides/_slim-select.scss

@@ -37,16 +37,6 @@ $spacing-s: $input-padding-x;
 .ss-main {
   color: $form-select-color;
 
-  &.is-invalid .ss-single-selected,
-  &.is-invalid .ss-multi-selected {
-    border-color: $form-feedback-icon-invalid-color;
-  }
-
-  &.is-valid .ss-single-selected,
-  &.is-valid .ss-multi-selected {
-    border-color: $form-feedback-icon-valid-color;
-  }
-
   .ss-single-selected,
   .ss-multi-selected {
     padding: $form-select-padding-y $input-padding-x $form-select-padding-y $form-select-padding-x;
@@ -195,3 +185,11 @@ $spacing-s: $input-padding-x;
 		}
 	}
 }
+
+// Apply red border for fields inside a row with .has-errors
+.has-errors {
+  .ss-single-selected,
+  .ss-multi-selected {
+    border-color: $red;
+  }
+}

netbox/project-static/styles/transitional/_forms.scss

@@ -16,3 +16,12 @@ form.object-edit {
     content: '\f06C4';
   }
 }
+
+// Set red border on form fields inside a row with .has-errors
+.has-errors {
+  input,
+  select,
+  textarea {
+    border: 1px solid $red;
+  }
+}

netbox/utilities/forms/mixins.py

@@ -51,19 +51,6 @@ class BootstrapMixin:
             # if 'placeholder' not in field.widget.attrs and field.label is not None:
             #     field.widget.attrs['placeholder'] = field.label
 
-    def is_valid(self):
-        is_valid = super().is_valid()
-
-        # Apply is-invalid CSS class to fields with errors
-        if not is_valid:
-            for field_name in self.errors:
-                # Ignore e.g. __all__
-                if field := self.fields.get(field_name):
-                    css = field.widget.attrs.get('class', '')
-                    field.widget.attrs['class'] = f'{css} is-invalid'
-
-        return is_valid
-
 
 class CheckLastUpdatedMixin(forms.Form):
     """