Auto merge of #17821 - bzbarsky:pseudo-prop-restrictions, r=emilio

Implement restrictions on which properties apply to which pseudo-elements

::first-line, ::first-letter, and ::placeholder don't allow all properties to apply to them.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix https://bugzilla.mozilla.org/show_bug.cgi?id=1382786

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because they're Gecko-only for the moment, and Gecko has tests.

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

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---
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components/style/gecko/pseudo_element.rs

@@ -10,6 +10,8 @@
 
 use cssparser::{ToCss, serialize_identifier};
 use gecko_bindings::structs::{self, CSSPseudoElementType};
+use properties::{PropertyFlags, APPLIES_TO_FIRST_LETTER, APPLIES_TO_FIRST_LINE};
+use properties::APPLIES_TO_PLACEHOLDER;
 use selector_parser::{NonTSPseudoClass, PseudoElementCascadeType, SelectorImpl};
 use std::fmt;
 use string_cache::Atom;
@@ -119,4 +121,15 @@ impl PseudoElement {
             _ => self.clone(),
         }
     }
+
+    /// Property flag that properties must have to apply to this pseudo-element.
+    #[inline]
+    pub fn property_restriction(&self) -> Option<PropertyFlags> {
+        match *self {
+            PseudoElement::FirstLetter => Some(APPLIES_TO_FIRST_LETTER),
+            PseudoElement::FirstLine => Some(APPLIES_TO_FIRST_LINE),
+            PseudoElement::Placeholder => Some(APPLIES_TO_PLACEHOLDER),
+            _ => None,
+        }
+    }
 }

components/style/properties/longhand/background.mako.rs

@@ -12,7 +12,8 @@ ${helpers.predefined_type("background-color", "Color",
     spec="https://drafts.csswg.org/css-backgrounds/#background-color",
     animation_value_type="IntermediateColor",
     ignored_when_colors_disabled=True,
-    allow_quirks=True)}
+    allow_quirks=True,
+    flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER")}
 
 ${helpers.predefined_type("background-image", "ImageLayer",
     initial_value="Either::First(None_)",
@@ -21,7 +22,8 @@ ${helpers.predefined_type("background-image", "ImageLayer",
     vector="True",
     animation_value_type="discrete",
     has_uncacheable_values="True" if product == "gecko" else "False",
-    ignored_when_colors_disabled="True")}
+    ignored_when_colors_disabled="True",
+    flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER")}
 
 % for (axis, direction, initial) in [("x", "Horizontal", "left"), ("y", "Vertical", "top")]:
     ${helpers.predefined_type(
@@ -32,11 +34,13 @@ ${helpers.predefined_type("background-image", "ImageLayer",
         spec="https://drafts.csswg.org/css-backgrounds-4/#propdef-background-position-" + axis,
         animation_value_type="ComputedValue",
         vector=True,
+        flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
     )}
 % endfor
 
 <%helpers:vector_longhand name="background-repeat" animation_value_type="discrete"
-                          spec="https://drafts.csswg.org/css-backgrounds/#the-background-repeat">
+                          spec="https://drafts.csswg.org/css-backgrounds/#the-background-repeat"
+                          flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER">
     use std::fmt;
     use style_traits::ToCss;
 
@@ -138,7 +142,8 @@ ${helpers.single_keyword("background-attachment",
                          vector=True,
                          gecko_constant_prefix="NS_STYLE_IMAGELAYER_ATTACHMENT",
                          spec="https://drafts.csswg.org/css-backgrounds/#the-background-attachment",
-                         animation_value_type="discrete")}
+                         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",
@@ -146,14 +151,16 @@ ${helpers.single_keyword("background-clip",
                          vector=True, extra_prefixes="webkit",
                          gecko_enum_prefix="StyleGeometryBox",
                          spec="https://drafts.csswg.org/css-backgrounds/#the-background-clip",
-                         animation_value_type="discrete")}
+                         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")}
+                         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::LengthOrPercentageOrAuto::Auto.into()",
@@ -161,6 +168,7 @@ ${helpers.predefined_type("background-size", "BackgroundSize",
     spec="https://drafts.csswg.org/css-backgrounds/#the-background-size",
     vector=True,
     animation_value_type="ComputedValue",
+    flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
     extra_prefixes="webkit")}
 
 // https://drafts.fxtf.org/compositing/#background-blend-mode
@@ -170,4 +178,5 @@ ${helpers.single_keyword("background-blend-mode",
                             saturation color luminosity""",
                          gecko_constant_prefix="NS_STYLE_BLEND",
                          vector=True, products="gecko", animation_value_type="discrete",
-                         spec="https://drafts.fxtf.org/compositing/#background-blend-mode")}
+                         spec="https://drafts.fxtf.org/compositing/#background-blend-mode",
+                         flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER")}

components/style/properties/longhand/border.mako.rs

@@ -27,12 +27,14 @@
                               animation_value_type="IntermediateColor",
                               logical=is_logical,
                               allow_quirks=not is_logical,
+                              flags="APPLIES_TO_FIRST_LETTER",
                               ignored_when_colors_disabled=True)}
 
     ${helpers.predefined_type("border-%s-style" % side_name, "BorderStyle",
                               "specified::BorderStyle::none",
                               alias=maybe_moz_logical_alias(product, side, "-moz-border-%s-style"),
                               spec=maybe_logical_spec(side, "style"),
+                              flags="APPLIES_TO_FIRST_LETTER",
                               animation_value_type="discrete" if not is_logical else "none",
                               logical=is_logical)}
 
@@ -44,6 +46,7 @@
                               spec=maybe_logical_spec(side, "width"),
                               animation_value_type="ComputedValue",
                               logical=is_logical,
+                              flags="APPLIES_TO_FIRST_LETTER",
                               allow_quirks=not is_logical)}
 % endfor
 
@@ -58,6 +61,7 @@ ${helpers.gecko_keyword_conversion(Keyword('border-style',
                               "parse", extra_prefixes="webkit",
                               spec="https://drafts.csswg.org/css-backgrounds/#border-%s-radius" % corner,
                               boxed=True,
+                              flags="APPLIES_TO_FIRST_LETTER",
                               animation_value_type="ComputedValue")}
 % endfor
 
@@ -67,6 +71,7 @@ ${helpers.gecko_keyword_conversion(Keyword('border-style',
     <%helpers:longhand name="-moz-border-${side}-colors" animation_value_type="discrete"
                        spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/-moz-border-*-colors)"
                        products="gecko"
+                       flags="APPLIES_TO_FIRST_LETTER"
                        ignored_when_colors_disabled="True">
         use std::fmt;
         use style_traits::ToCss;
@@ -206,6 +211,7 @@ ${helpers.predefined_type("border-image-source", "ImageLayer",
     vector=False,
     animation_value_type="discrete",
     has_uncacheable_values=False,
+    flags="APPLIES_TO_FIRST_LETTER",
     boxed="True")}
 
 ${helpers.predefined_type("border-image-outset", "LengthOrNumberRect",
@@ -214,9 +220,11 @@ ${helpers.predefined_type("border-image-outset", "LengthOrNumberRect",
     initial_specified_value="specified::LengthOrNumber::zero().into()",
     spec="https://drafts.csswg.org/css-backgrounds/#border-image-outset",
     animation_value_type="discrete",
+    flags="APPLIES_TO_FIRST_LETTER",
     boxed=True)}
 
 <%helpers:longhand name="border-image-repeat" animation_value_type="discrete"
+                   flags="APPLIES_TO_FIRST_LETTER"
                    spec="https://drafts.csswg.org/css-backgrounds/#border-image-repeat">
     use style_traits::ToCss;
 
@@ -278,6 +286,7 @@ ${helpers.predefined_type("border-image-width", "BorderImageWidth",
     initial_specified_value="specified::BorderImageSideWidth::one().into()",
     spec="https://drafts.csswg.org/css-backgrounds/#border-image-width",
     animation_value_type="discrete",
+    flags="APPLIES_TO_FIRST_LETTER",
     boxed=True)}
 
 ${helpers.predefined_type("border-image-slice", "BorderImageSlice",
@@ -285,6 +294,7 @@ ${helpers.predefined_type("border-image-slice", "BorderImageSlice",
     initial_specified_value="specified::NumberOrPercentage::Percentage(specified::Percentage::new(1.)).into()",
     spec="https://drafts.csswg.org/css-backgrounds/#border-image-slice",
     animation_value_type="discrete",
+    flags="APPLIES_TO_FIRST_LETTER",
     boxed=True)}
 
 #[cfg(feature = "gecko")]

components/style/properties/longhand/box.mako.rs

@@ -10,10 +10,14 @@
                          gecko_name="Display") %>
 
 // TODO(SimonSapin): don't parse `inline-table`, since we don't support it
+//
+// We allow "display" to apply to placeholders because we need to make the
+// placeholder pseudo-element an inline-block in the UA stylesheet in Gecko.
 <%helpers:longhand name="display"
                    need_clone="True"
                    animation_value_type="discrete"
                    custom_cascade="${product == 'servo'}"
+                   flags="APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-display/#propdef-display">
     <%
         values = """inline block inline-block
@@ -216,6 +220,7 @@ ${helpers.single_keyword("position", "static absolute relative fixed",
                                   gecko_inexhaustive="True"
                                   gecko_ffi_name="mFloat"
                                   gecko_pref_ident="float_"
+                                  flags="APPLIES_TO_FIRST_LETTER"
                                   spec="https://drafts.csswg.org/css-box/#propdef-float">
     no_viewport_percentage!(SpecifiedValue);
     impl ToComputedValue for SpecifiedValue {
@@ -306,6 +311,7 @@ ${helpers.single_keyword("position", "static absolute relative fixed",
 </%helpers:longhand>
 
 <%helpers:longhand name="vertical-align" animation_value_type="ComputedValue"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                    spec="https://www.w3.org/TR/CSS2/visudet.html#propdef-vertical-align">
     use std::fmt;
     use style_traits::ToCss;
@@ -432,6 +438,7 @@ ${helpers.single_keyword("-servo-overflow-clip-box", "padding-box content-box",
 
 ${helpers.single_keyword("overflow-clip-box", "padding-box content-box",
     products="gecko", animation_value_type="discrete", internal=True,
+    flags="APPLIES_TO_PLACEHOLDER",
     spec="Internal, not web-exposed, \
           may be standardized in the future (https://developer.mozilla.org/en-US/docs/Web/CSS/overflow-clip-box)")}
 
@@ -445,10 +452,12 @@ ${helpers.single_keyword("overflow-x", "visible hidden scroll auto",
                          extra_gecko_values="-moz-hidden-unscrollable",
                          custom_consts=overflow_custom_consts,
                          gecko_constant_prefix="NS_STYLE_OVERFLOW",
+                         flags="APPLIES_TO_PLACEHOLDER",
                          spec="https://drafts.csswg.org/css-overflow/#propdef-overflow-x")}
 
 // FIXME(pcwalton, #2742): Implement scrolling for `scroll` and `auto`.
 <%helpers:longhand name="overflow-y" need_clone="True" animation_value_type="discrete"
+                   flags="APPLIES_TO_PLACEHOLDER",
                    spec="https://drafts.csswg.org/css-overflow/#propdef-overflow-y">
     pub use super::overflow_x::{SpecifiedValue, parse, get_initial_value, computed_value};
 </%helpers:longhand>
@@ -1618,10 +1627,15 @@ ${helpers.single_keyword("page-break-inside",
 // CSS Basic User Interface Module Level 3
 // http://dev.w3.org/csswg/css-ui
 // FIXME support logical values `block` and `inline` (https://drafts.csswg.org/css-logical-props/#resize)
+//
+// This is APPLIES_TO_PLACEHOLDER so we can override, in the UA sheet, the
+// 'resize' property we'd inherit from textarea otherwise.  Basically, just
+// makes the UA rules easier to write.
 ${helpers.single_keyword("resize",
                          "none both horizontal vertical",
                          products="gecko",
                          spec="https://drafts.csswg.org/css-ui/#propdef-resize",
+                         flags="APPLIES_TO_PLACEHOLDER",
                          animation_value_type="discrete")}
 
 
@@ -1888,6 +1902,7 @@ ${helpers.predefined_type("shape-outside", "basic_shape::FloatAreaShape",
                           "generics::basic_shape::ShapeSource::None",
                           products="gecko", boxed="True",
                           animation_value_type="none",
+                          flags="APPLIES_TO_FIRST_LETTER",
                           spec="https://drafts.csswg.org/css-shapes/#shape-outside-property")}
 
 <%helpers:longhand name="touch-action"

components/style/properties/longhand/color.mako.rs

@@ -11,6 +11,7 @@
 <%helpers:longhand name="color" need_clone="True"
                    animation_value_type="IntermediateRGBA"
                    ignored_when_colors_disabled="True"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-color/#color">
     use cssparser::RGBA;
     use values::specified::{AllowQuirks, Color};

components/style/properties/longhand/effects.mako.rs

@@ -11,7 +11,7 @@ ${helpers.predefined_type("opacity",
                           "Opacity",
                           "1.0",
                           animation_value_type="ComputedValue",
-                          flags="CREATES_STACKING_CONTEXT",
+                          flags="CREATES_STACKING_CONTEXT APPLIES_TO_PLACEHOLDER",
                           spec="https://drafts.csswg.org/css-color/#opacity")}
 
 ${helpers.predefined_type(
@@ -22,6 +22,7 @@ ${helpers.predefined_type(
     animation_value_type="AnimatedBoxShadowList",
     extra_prefixes="webkit",
     ignored_when_colors_disabled=True,
+    flags="APPLIES_TO_FIRST_LETTER",
     spec="https://drafts.csswg.org/css-backgrounds/#box-shadow",
 )}
 

components/style/properties/longhand/font.mako.rs

@@ -69,6 +69,7 @@ macro_rules! impl_gecko_keyword_conversions {
 </%def>
 
 <%helpers:longhand name="font-family" animation_value_type="discrete" need_index="True"  boxed="${product == 'gecko'}"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-fonts/#propdef-font-family">
     use properties::longhands::system_font::SystemFont;
     use self::computed_value::{FontFamily, FamilyName};
@@ -379,6 +380,7 @@ ${helpers.single_keyword_system("font-style",
                                 gecko_constant_prefix="NS_FONT_STYLE",
                                 gecko_ffi_name="mFont.style",
                                 spec="https://drafts.csswg.org/css-fonts/#propdef-font-style",
+                                flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                                 animation_value_type="discrete")}
 
 
@@ -395,9 +397,11 @@ ${helpers.single_keyword_system("font-variant-caps",
                                gecko_ffi_name="mFont.variantCaps",
                                spec="https://drafts.csswg.org/css-fonts/#propdef-font-variant-caps",
                                custom_consts=font_variant_caps_custom_consts,
+                               flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                                animation_value_type="discrete")}
 
 <%helpers:longhand name="font-weight" need_clone="True" animation_value_type="ComputedValue"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-fonts/#propdef-font-weight">
     use properties::longhands::system_font::SystemFont;
 
@@ -559,6 +563,7 @@ ${helpers.single_keyword_system("font-variant-caps",
 </%helpers:longhand>
 
 <%helpers:longhand name="font-size" need_clone="True" animation_value_type="ComputedValue"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    allow_quirks="True" spec="https://drafts.csswg.org/css-fonts/#propdef-font-size">
     use app_units::Au;
     use properties::longhands::system_font::SystemFont;
@@ -1020,6 +1025,7 @@ ${helpers.single_keyword_system("font-variant-caps",
 </%helpers:longhand>
 
 <%helpers:longhand products="gecko" name="font-size-adjust" animation_value_type="ComputedValue"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-fonts/#propdef-font-size-adjust">
     use properties::longhands::system_font::SystemFont;
 
@@ -1141,6 +1147,7 @@ ${helpers.single_keyword_system("font-variant-caps",
 </%helpers:longhand>
 
 <%helpers:longhand products="gecko" name="font-synthesis" animation_value_type="discrete"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-fonts/#propdef-font-synthesis">
     use std::fmt;
     use style_traits::ToCss;
@@ -1238,6 +1245,7 @@ ${helpers.single_keyword_system("font-stretch",
                                 gecko_constant_prefix="NS_FONT_STRETCH",
                                 cast_type='i16',
                                 spec="https://drafts.csswg.org/css-fonts/#propdef-font-stretch",
+                                flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                                 animation_value_type="ComputedValue")}
 
 ${helpers.single_keyword_system("font-kerning",
@@ -1246,9 +1254,11 @@ ${helpers.single_keyword_system("font-kerning",
                                 gecko_ffi_name="mFont.kerning",
                                 gecko_constant_prefix="NS_FONT_KERNING",
                                 spec="https://drafts.csswg.org/css-fonts/#propdef-font-kerning",
+                                flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                                 animation_value_type="discrete")}
 
 <%helpers:longhand name="font-variant-alternates" products="gecko" animation_value_type="none"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-fonts/#propdef-font-variant-alternates">
     use properties::longhands::system_font::SystemFont;
     use std::fmt;
@@ -1458,6 +1468,7 @@ macro_rules! exclusive_value {
 }
 
 <%helpers:longhand name="font-variant-east-asian" products="gecko" animation_value_type="discrete"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-fonts/#propdef-font-variant-east-asian">
     use properties::longhands::system_font::SystemFont;
     use std::fmt;
@@ -1603,6 +1614,7 @@ macro_rules! exclusive_value {
 </%helpers:longhand>
 
 <%helpers:longhand name="font-variant-ligatures" products="gecko" animation_value_type="discrete"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-fonts/#propdef-font-variant-ligatures">
     use properties::longhands::system_font::SystemFont;
     use std::fmt;
@@ -1762,6 +1774,7 @@ macro_rules! exclusive_value {
 </%helpers:longhand>
 
 <%helpers:longhand name="font-variant-numeric" products="gecko" animation_value_type="discrete"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-fonts/#propdef-font-variant-numeric">
     use properties::longhands::system_font::SystemFont;
     use std::fmt;
@@ -1915,10 +1928,12 @@ ${helpers.single_keyword_system("font-variant-position",
                                 gecko_ffi_name="mFont.variantPosition",
                                 gecko_constant_prefix="NS_FONT_VARIANT_POSITION",
                                 spec="https://drafts.csswg.org/css-fonts/#propdef-font-variant-position",
+                                flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                                 animation_value_type="discrete")}
 
 <%helpers:longhand name="font-feature-settings" products="gecko" animation_value_type="discrete"
                    extra_prefixes="moz" boxed="True"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-fonts/#propdef-font-feature-settings">
     use properties::longhands::system_font::SystemFont;
     use values::generics::FontSettings;
@@ -1961,6 +1976,7 @@ https://drafts.csswg.org/css-fonts-4/#low-level-font-variation-settings-control-
 """
 %>
 <%helpers:longhand name="font-variation-settings" products="gecko" animation_value_type="none"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    spec="${variation_spec}">
     use values::computed::ComputedValueAsSpecified;
     use values::generics::FontSettings;
@@ -1991,6 +2007,7 @@ https://drafts.csswg.org/css-fonts-4/#low-level-font-variation-settings-control-
 
 <%helpers:longhand name="font-language-override" products="gecko" animation_value_type="discrete"
                    extra_prefixes="moz" boxed="True"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-fonts-3/#propdef-font-language-override">
     use properties::longhands::system_font::SystemFont;
     use std::fmt;
@@ -2550,6 +2567,7 @@ ${helpers.single_keyword("-moz-osx-font-smoothing",
                          gecko_ffi_name="mFont.smoothing",
                          products="gecko",
                          spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/font-smooth)",
+                         flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                          animation_value_type="discrete")}
 
 ${helpers.predefined_type("-moz-min-font-size-ratio",

components/style/properties/longhand/inherited_text.mako.rs

@@ -10,6 +10,7 @@ ${helpers.predefined_type("line-height",
                           "LineHeight",
                           "computed::LineHeight::normal()",
                           animation_value_type="ComputedValue",
+                          flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                           spec="https://drafts.csswg.org/css2/visudet.html#propdef-line-height")}
 
 // CSS Text Module Level 3
@@ -19,6 +20,7 @@ ${helpers.single_keyword("text-transform",
                          "none capitalize uppercase lowercase",
                          extra_gecko_values="full-width",
                          animation_value_type="discrete",
+                         flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                          spec="https://drafts.csswg.org/css-text/#propdef-text-transform")}
 
 ${helpers.single_keyword("hyphens", "manual none auto",
@@ -66,6 +68,7 @@ ${helpers.single_keyword("word-break",
                                   gecko_enum_prefix="StyleTextJustify"
                                   gecko_inexhaustive="True"
                                   animation_value_type="discrete"
+                                  flags="APPLIES_TO_PLACEHOLDER",
                                   spec="https://drafts.csswg.org/css-text/#propdef-text-justify">
     no_viewport_percentage!(SpecifiedValue);
 
@@ -109,6 +112,7 @@ ${helpers.single_keyword("text-align-last",
 
 // TODO make this a shorthand and implement text-align-last/text-align-all
 <%helpers:longhand name="text-align" animation_value_type="discrete"
+                   flags="APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-text/#propdef-text-align">
     no_viewport_percentage!(SpecifiedValue);
     pub mod computed_value {
@@ -267,12 +271,14 @@ ${helpers.predefined_type("letter-spacing",
                           "LetterSpacing",
                           "computed::LetterSpacing::normal()",
                           animation_value_type="ComputedValue",
+                          flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                           spec="https://drafts.csswg.org/css-text/#propdef-letter-spacing")}
 
 ${helpers.predefined_type("word-spacing",
                           "WordSpacing",
                           "computed::WordSpacing::normal()",
                           animation_value_type="ComputedValue",
+                          flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                           spec="https://drafts.csswg.org/css-text/#propdef-word-spacing")}
 
 <%helpers:longhand name="-servo-text-decorations-in-effect"
@@ -367,6 +373,9 @@ ${helpers.predefined_type("word-spacing",
                                   needs_conversion="True"
                                   gecko_inexhaustive="True"
                                   animation_value_type="discrete"
+                                  // Only allowed for UA sheets, which set it
+                                  // !important.
+                                  flags="APPLIES_TO_PLACEHOLDER"
                                   spec="https://drafts.csswg.org/css-text/#propdef-white-space">
     use values::computed::ComputedValueAsSpecified;
     impl ComputedValueAsSpecified for SpecifiedValue {}
@@ -414,6 +423,7 @@ ${helpers.predefined_type(
     vector=True,
     animation_value_type="AnimatedTextShadowList",
     ignored_when_colors_disabled=True,
+    flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
     spec="https://drafts.csswg.org/css-text-decor-3/#text-shadow-property",
 )}
 
@@ -718,6 +728,7 @@ ${helpers.predefined_type(
     "computed_value::T::currentcolor()",
     products="gecko", animation_value_type="IntermediateColor",
     need_clone=True, ignored_when_colors_disabled=True,
+    flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
     spec="https://compat.spec.whatwg.org/#the-webkit-text-fill-color")}
 
 ${helpers.predefined_type(
@@ -726,6 +737,7 @@ ${helpers.predefined_type(
     initial_specified_value="specified::Color::currentcolor()",
     products="gecko", animation_value_type="IntermediateColor",
     need_clone=True, ignored_when_colors_disabled=True,
+    flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
     spec="https://compat.spec.whatwg.org/#the-webkit-text-stroke-color")}
 
 ${helpers.predefined_type("-webkit-text-stroke-width",
@@ -734,6 +746,7 @@ ${helpers.predefined_type("-webkit-text-stroke-width",
                           initial_specified_value="specified::BorderSideWidth::Length(specified::Length::zero())",
                           computed_type="::app_units::Au",
                           products="gecko",
+                          flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                           spec="https://compat.spec.whatwg.org/#the-webkit-text-stroke-width",
                           animation_value_type="none")}
 

components/style/properties/longhand/margin.mako.rs

@@ -17,5 +17,6 @@
                               alias=maybe_moz_logical_alias(product, side, "-moz-margin-%s"),
                               allow_quirks=not side[1],
                               animation_value_type="ComputedValue", logical = side[1], spec = spec,
+                              flags="APPLIES_TO_FIRST_LETTER",
                               allowed_in_page_rule=True)}
 % endfor

components/style/properties/longhand/padding.mako.rs

@@ -6,6 +6,8 @@
 <% from data import ALL_SIDES, maybe_moz_logical_alias %>
 <% data.new_style_struct("Padding", inherited=False) %>
 
+// APPLIES_TO_PLACEHOLDER so we can set it in UA  stylesheets.  But we use a
+// !important value there, so pages can't set it.
 % for side in ALL_SIDES:
     <%
         spec = "https://drafts.csswg.org/css-box/#propdef-padding-%s" % side[0]
@@ -19,5 +21,6 @@
                               animation_value_type="ComputedValue",
                               logical = side[1],
                               spec = spec,
+                              flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_PLACEHOLDER",
                               allow_quirks=not side[1])}
 % endfor

components/style/properties/longhand/pointing.mako.rs

@@ -152,6 +152,7 @@
 // TODO(pcwalton): SVG-only values.
 ${helpers.single_keyword("pointer-events", "auto none", animation_value_type="discrete",
                          extra_gecko_values="visiblepainted visiblefill visiblestroke visible painted fill stroke all",
+                         flags="APPLIES_TO_PLACEHOLDER",
                          spec="https://www.w3.org/TR/SVG11/interact.html#PointerEventsProperty")}
 
 ${helpers.single_keyword("-moz-user-input", "auto none enabled disabled",

components/style/properties/longhand/text.mako.rs

@@ -13,6 +13,7 @@
                                              Method("has_line_through", "bool")]) %>
 
 <%helpers:longhand name="text-overflow" animation_value_type="discrete" boxed="True"
+                   flags="APPLIES_TO_PLACEHOLDER"
                    spec="https://drafts.csswg.org/css-ui/#propdef-text-overflow">
     use std::fmt;
     use style_traits::ToCss;
@@ -142,6 +143,7 @@ ${helpers.single_keyword("unicode-bidi",
                    custom_cascade="${product == 'servo'}"
                    need_clone=True
                    animation_value_type="discrete"
+                   flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                    spec="https://drafts.csswg.org/css-text-decor/#propdef-text-decoration-line">
     use std::fmt;
     use style_traits::ToCss;
@@ -262,6 +264,7 @@ ${helpers.single_keyword("text-decoration-style",
                          "solid double dotted dashed wavy -moz-none",
                          products="gecko",
                          animation_value_type="discrete",
+                         flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
                          spec="https://drafts.csswg.org/css-text-decor/#propdef-text-decoration-style")}
 
 ${helpers.predefined_type(
@@ -271,6 +274,7 @@ ${helpers.predefined_type(
     products="gecko",
     animation_value_type="IntermediateColor",
     ignored_when_colors_disabled=True,
+    flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",
     spec="https://drafts.csswg.org/css-text-decor/#propdef-text-decoration-color")}
 
 ${helpers.predefined_type(
@@ -280,4 +284,5 @@ ${helpers.predefined_type(
     initial_specified_value="specified::InitialLetter::normal()",
     animation_value_type="discrete",
     products="gecko",
+    flags="APPLIES_TO_FIRST_LETTER",
     spec="https://drafts.csswg.org/css-inline/#sizing-drop-initials")}

components/style/properties/properties.mako.rs

@@ -442,6 +442,12 @@ bitflags! {
         const ABSPOS_CB = 1 << 2,
         /// This shorthand property is an alias of another property.
         const SHORTHAND_ALIAS_PROPERTY = 1 << 3,
+        /// This longhand property applies to ::first-letter.
+        const APPLIES_TO_FIRST_LETTER = 1 << 4,
+        /// This longhand property applies to ::first-line.
+        const APPLIES_TO_FIRST_LINE = 1 << 5,
+        /// This longhand property applies to ::placeholder.
+        const APPLIES_TO_PLACEHOLDER = 1 << 6,
     }
 }
 
@@ -2866,11 +2872,26 @@ pub fn cascade(
                 &[]
             };
             let node_importance = node.importance();
+
+            let property_restriction = pseudo.and_then(|p| p.property_restriction());
+
             declarations
                 .iter()
                 // Yield declarations later in source order (with more precedence) first.
                 .rev()
                 .filter_map(move |&(ref declaration, declaration_importance)| {
+                    if let Some(property_restriction) = property_restriction {
+                        // declaration.id() is either a longhand or a custom
+                        // property.  Custom properties are always allowed, but
+                        // longhands are only allowed if they have our
+                        // property_restriction flag set.
+                        if let PropertyDeclarationId::Longhand(id) = declaration.id() {
+                            if !id.flags().contains(property_restriction) {
+                                return None
+                            }
+                        }
+                    }
+
                     if declaration_importance == node_importance {
                         Some((declaration, cascade_level))
                     } else {

components/style/servo/selector_parser.rs

@@ -13,6 +13,7 @@ use dom::{OpaqueNode, TElement, TNode};
 use element_state::ElementState;
 use fnv::FnvHashMap;
 use invalidation::element::element_wrapper::ElementSnapshot;
+use properties::PropertyFlags;
 use selector_parser::{AttrValue as SelectorAttrValue, ElementExt, PseudoElementCascadeType, SelectorParser};
 use selectors::Element;
 use selectors::attr::{AttrSelectorOperation, NamespaceConstraint, CaseSensitivity};
@@ -39,6 +40,12 @@ pub enum PseudoElement {
     Before,
     Selection,
     // If/when :first-letter is added, update is_first_letter accordingly.
+
+    // If/when ::first-letter, ::first-line, or ::placeholder are added, adjust
+    // our property_restriction implementation to do property filtering for
+    // them.  Also, make sure the UA sheet has the !important rules some of the
+    // APPLIES_TO_PLACEHOLDER properties expect!
+
     // Non-eager pseudos.
     DetailsSummary,
     DetailsContent,
@@ -169,6 +176,12 @@ impl PseudoElement {
 
     /// Stub, only Gecko needs this
     pub fn pseudo_info(&self) { () }
+
+    /// Property flag that properties must have to apply to this pseudo-element.
+    #[inline]
+    pub fn property_restriction(&self) -> Option<PropertyFlags> {
+        None
+    }
 }
 
 /// The type used for storing pseudo-class string arguments.