--- a/servo/components/style/properties/gecko.mako.rs
+++ b/servo/components/style/properties/gecko.mako.rs
@@ -3800,17 +3800,19 @@ fn static_assert() {
                         % for value in longhand.keyword.values_for("gecko"):
                         % if keyword.needs_cast():
                         ${keyword.casted_constant_name(value, "u8")}
                         % else:
                         structs::${keyword.gecko_constant(value)}
                         % endif
                             => Keyword::${to_camel_case(value)},
                         % endfor
+                        % if keyword.gecko_inexhaustive:
                         _ => panic!("Found unexpected value in style struct for ${ident} property"),
+                        % endif
                     }
                 }).collect()
         )
     }
 </%def>
 
 <%def name="impl_common_image_layer_properties(shorthand)">
     <%

--- a/servo/components/style/properties/longhand/background.mako.rs
+++ b/servo/components/style/properties/longhand/background.mako.rs
@@ -55,32 +55,38 @@
 ${helpers.single_keyword("background-attachment",
                          "scroll fixed" + (" local" if product == "gecko" else ""),
                          vector=True,
                          gecko_enum_prefix="StyleImageLayerAttachment",
                          spec="https://drafts.csswg.org/css-backgrounds/#the-background-attachment",
                          animation_value_type="discrete",
                          flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER")}
 
-${helpers.single_keyword("background-clip",
-                         "border-box padding-box content-box",
-                         extra_gecko_values="text",
-                         vector=True, extra_prefixes="webkit",
-                         gecko_enum_prefix="StyleGeometryBox",
-                         spec="https://drafts.csswg.org/css-backgrounds/#the-background-clip",
-                         animation_value_type="discrete",
-                         flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER")}
+${helpers.single_keyword(
+    "background-clip",
+    "border-box padding-box content-box",
+    extra_gecko_values="text",
+    vector=True, extra_prefixes="webkit",
+    gecko_enum_prefix="StyleGeometryBox",
+    gecko_inexhaustive=True,
+    spec="https://drafts.csswg.org/css-backgrounds/#the-background-clip",
+    animation_value_type="discrete",
+    flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
+)}
 
-${helpers.single_keyword("background-origin",
-                         "padding-box border-box content-box",
-                         vector=True, extra_prefixes="webkit",
-                         gecko_enum_prefix="StyleGeometryBox",
-                         spec="https://drafts.csswg.org/css-backgrounds/#the-background-origin",
-                         animation_value_type="discrete",
-                         flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER")}
+${helpers.single_keyword(
+    "background-origin",
+    "padding-box border-box content-box",
+    vector=True, extra_prefixes="webkit",
+    gecko_enum_prefix="StyleGeometryBox",
+    gecko_inexhaustive=True,
+    spec="https://drafts.csswg.org/css-backgrounds/#the-background-origin",
+    animation_value_type="discrete",
+    flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
+)}
 
 ${helpers.predefined_type(
     "background-size",
     "BackgroundSize",
     initial_value="computed::BackgroundSize::auto()",
     initial_specified_value="specified::BackgroundSize::auto()",
     spec="https://drafts.csswg.org/css-backgrounds/#the-background-size",
     vector=True,

--- a/servo/components/style/properties/longhand/svg.mako.rs
+++ b/servo/components/style/properties/longhand/svg.mako.rs
@@ -103,35 +103,42 @@
         initial_specified_value="specified::PositionComponent::Center",
         spec="https://drafts.fxtf.org/css-masking/#propdef-mask-position",
         animation_value_type="ComputedValue",
         vector_animation_type="repeatable_list",
         vector=True,
     )}
 % endfor
 
-${helpers.single_keyword("mask-clip",
-                         "border-box content-box padding-box",
-                         extra_gecko_values="fill-box stroke-box view-box no-clip",
-                         vector=True,
-                         products="gecko",
-                         extra_prefixes="webkit",
-                         gecko_enum_prefix="StyleGeometryBox",
-                         animation_value_type="discrete",
-                         spec="https://drafts.fxtf.org/css-masking/#propdef-mask-clip")}
+${helpers.single_keyword(
+    "mask-clip",
+    "border-box content-box padding-box",
+    extra_gecko_values="fill-box stroke-box view-box no-clip",
+    vector=True,
+    products="gecko",
+    extra_prefixes="webkit",
+    gecko_enum_prefix="StyleGeometryBox",
+    gecko_inexhaustive=True,
+    animation_value_type="discrete",
+    spec="https://drafts.fxtf.org/css-masking/#propdef-mask-clip",
+)}
 
-${helpers.single_keyword("mask-origin",
-                         "border-box content-box padding-box",
-                         extra_gecko_values="fill-box stroke-box view-box",
-                         vector=True,
-                         products="gecko",
-                         extra_prefixes="webkit",
-                         gecko_enum_prefix="StyleGeometryBox",
-                         animation_value_type="discrete",
-                         spec="https://drafts.fxtf.org/css-masking/#propdef-mask-origin")}
+${helpers.single_keyword(
+    "mask-origin",
+    "border-box content-box padding-box",
+    extra_gecko_values="fill-box stroke-box view-box",
+    vector=True,
+    products="gecko",
+    extra_prefixes="webkit",
+    gecko_enum_prefix="StyleGeometryBox",
+    gecko_inexhaustive=True,
+    animation_value_type="discrete",
+    spec="https://drafts.fxtf.org/css-masking/#propdef-mask-origin",
+)}
+
 ${helpers.predefined_type(
     "mask-size",
     "background::BackgroundSize",
     "computed::BackgroundSize::auto()",
     initial_specified_value="specified::BackgroundSize::auto()",
     products="gecko",
     extra_prefixes="webkit",
     spec="https://drafts.fxtf.org/css-masking/#propdef-mask-size",