Add checkmark pseudo element

Adds a `::-servo-checkmark` pseudo element for checkbox and radio inputs.

Signed-off-by: Luke Warlow <lwarlow@igalia.com>

Author: lukewarlow

style/servo/selector_parser.rs

@@ -69,6 +69,7 @@ pub enum PseudoElement {
     // Private, Servo-specific implemented pseudos. Only matchable in UA sheet.
     ServoTextControlInnerContainer,
     ServoTextControlInnerEditor,
+    ServoCheckmark,
 
     // Other Servo-specific pseudos.
     ServoAnonymousBox,
@@ -98,6 +99,7 @@ impl ToCss for PseudoElement {
             Marker => "::marker",
             ColorSwatch => "::color-swatch",
             Placeholder => "::placeholder",
+            ServoCheckmark => "::-servo-checkmark",
             ServoTextControlInnerContainer => "::-servo-text-control-inner-container",
             ServoTextControlInnerEditor => "::-servo-text-control-inner-editor",
             ServoAnonymousBox => "::-servo-anonymous-box",
@@ -248,6 +250,7 @@ impl PseudoElement {
             | PseudoElement::DetailsSummary
             | PseudoElement::Marker
             | PseudoElement::Placeholder
+            | PseudoElement::ServoCheckmark
             | PseudoElement::ServoTextControlInnerContainer
             | PseudoElement::ServoTextControlInnerEditor => PseudoElementCascadeType::Lazy,
             PseudoElement::DetailsContent
@@ -632,6 +635,12 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
             "backdrop" => Backdrop,
             "selection" => Selection,
             "marker" => Marker,
+            "-servo-checkmark" => {
+                if !self.in_user_agent_stylesheet() {
+                    return Err(location.new_custom_error(SelectorParseErrorKind::UnexpectedIdent(name.clone())))
+                }
+                ServoCheckmark
+            },
             "-servo-details-summary" => {
                 if !self.in_user_agent_stylesheet() {
                     return Err(location.new_custom_error(SelectorParseErrorKind::UnexpectedIdent(name.clone())))