Support the `<meter>` element (#35524)

* Allow attaching UA shadow roots to any element

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

* Implement a UA shadow tree for the <meter> element

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

* Add UA styles for the meter element

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

* Include spec text when computing meter state

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

---------

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

6 files changed, 207 insertions, 27 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 591883b31d7..8bd2883fe7a 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1017,7 +1017,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "117725a109d387c937a1533ce01b450cbde6b88abceea8473c4d7a85853cda3c"
 dependencies = [
  "lazy_static",
- "windows-sys 0.59.0",
+ "windows-sys 0.48.0",
 ]
 
 [[package]]
@@ -1686,7 +1686,7 @@ dependencies = [
 [[package]]
 name = "dom"
 version = "0.0.1"
-source = "git+https://github.com/servo/stylo?branch=2025-02-03#1977193ab231d89cf0f87ccc23c05e4ff67ee3fd"
+source = "git+https://github.com/servo/stylo?branch=2025-02-03#7eaabe1687ea6afa84c6d7d6b8c1b3466aa56a36"
 dependencies = [
  "bitflags 2.8.0",
  "malloc_size_of",
@@ -1967,7 +1967,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "33d852cb9b869c2a9b3df2f71a3074817f01e1844f839a144f5fcef059a4eb5d"
 dependencies = [
  "libc",
- "windows-sys 0.59.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -2493,7 +2493,7 @@ dependencies = [
  "gobject-sys",
  "libc",
  "system-deps",
- "windows-sys 0.59.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -3955,7 +3955,7 @@ checksum = "e19b23d53f35ce9f56aebc7d1bb4e6ac1e9c0db7ac85c8d1760c04379edced37"
 dependencies = [
  "hermit-abi 0.4.0",
  "libc",
- "windows-sys 0.59.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -4299,7 +4299,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fc2f4eb4bc735547cfed7c0a4922cbd04a4655978c09b54f1f7b228750664c34"
 dependencies = [
  "cfg-if",
- "windows-targets 0.52.6",
+ "windows-targets 0.48.5",
 ]
 
 [[package]]
@@ -4477,7 +4477,7 @@ dependencies = [
 [[package]]
 name = "malloc_size_of"
 version = "0.0.1"
-source = "git+https://github.com/servo/stylo?branch=2025-02-03#1977193ab231d89cf0f87ccc23c05e4ff67ee3fd"
+source = "git+https://github.com/servo/stylo?branch=2025-02-03#7eaabe1687ea6afa84c6d7d6b8c1b3466aa56a36"
 dependencies = [
  "app_units",
  "cssparser",
@@ -6172,7 +6172,7 @@ dependencies = [
  "errno",
  "libc",
  "linux-raw-sys",
- "windows-sys 0.59.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -6481,7 +6481,7 @@ dependencies = [
 [[package]]
 name = "selectors"
 version = "0.26.0"
-source = "git+https://github.com/servo/stylo?branch=2025-02-03#1977193ab231d89cf0f87ccc23c05e4ff67ee3fd"
+source = "git+https://github.com/servo/stylo?branch=2025-02-03#7eaabe1687ea6afa84c6d7d6b8c1b3466aa56a36"
 dependencies = [
  "bitflags 2.8.0",
  "cssparser",
@@ -6766,7 +6766,7 @@ dependencies = [
 [[package]]
 name = "servo_arc"
 version = "0.4.0"
-source = "git+https://github.com/servo/stylo?branch=2025-02-03#1977193ab231d89cf0f87ccc23c05e4ff67ee3fd"
+source = "git+https://github.com/servo/stylo?branch=2025-02-03#7eaabe1687ea6afa84c6d7d6b8c1b3466aa56a36"
 dependencies = [
  "serde",
  "stable_deref_trait",
@@ -6775,7 +6775,7 @@ dependencies = [
 [[package]]
 name = "servo_atoms"
 version = "0.0.1"
-source = "git+https://github.com/servo/stylo?branch=2025-02-03#1977193ab231d89cf0f87ccc23c05e4ff67ee3fd"
+source = "git+https://github.com/servo/stylo?branch=2025-02-03#7eaabe1687ea6afa84c6d7d6b8c1b3466aa56a36"
 dependencies = [
  "string_cache",
  "string_cache_codegen",
@@ -7133,7 +7133,7 @@ checksum = "a2eb9349b6444b326872e140eb1cf5e7c522154d69e7a0ffb0fb81c06b37543f"
 [[package]]
 name = "static_prefs"
 version = "0.1.0"
-source = "git+https://github.com/servo/stylo?branch=2025-02-03#1977193ab231d89cf0f87ccc23c05e4ff67ee3fd"
+source = "git+https://github.com/servo/stylo?branch=2025-02-03#7eaabe1687ea6afa84c6d7d6b8c1b3466aa56a36"
 
 [[package]]
 name = "strck"
@@ -7213,7 +7213,7 @@ dependencies = [
 [[package]]
 name = "style"
 version = "0.0.1"
-source = "git+https://github.com/servo/stylo?branch=2025-02-03#1977193ab231d89cf0f87ccc23c05e4ff67ee3fd"
+source = "git+https://github.com/servo/stylo?branch=2025-02-03#7eaabe1687ea6afa84c6d7d6b8c1b3466aa56a36"
 dependencies = [
  "app_units",
  "arrayvec",
@@ -7271,7 +7271,7 @@ dependencies = [
 [[package]]
 name = "style_config"
 version = "0.0.1"
-source = "git+https://github.com/servo/stylo?branch=2025-02-03#1977193ab231d89cf0f87ccc23c05e4ff67ee3fd"
+source = "git+https://github.com/servo/stylo?branch=2025-02-03#7eaabe1687ea6afa84c6d7d6b8c1b3466aa56a36"
 dependencies = [
  "lazy_static",
 ]
@@ -7279,7 +7279,7 @@ dependencies = [
 [[package]]
 name = "style_derive"
 version = "0.0.1"
-source = "git+https://github.com/servo/stylo?branch=2025-02-03#1977193ab231d89cf0f87ccc23c05e4ff67ee3fd"
+source = "git+https://github.com/servo/stylo?branch=2025-02-03#7eaabe1687ea6afa84c6d7d6b8c1b3466aa56a36"
 dependencies = [
  "darling",
  "proc-macro2",
@@ -7309,7 +7309,7 @@ dependencies = [
 [[package]]
 name = "style_traits"
 version = "0.0.1"
-source = "git+https://github.com/servo/stylo?branch=2025-02-03#1977193ab231d89cf0f87ccc23c05e4ff67ee3fd"
+source = "git+https://github.com/servo/stylo?branch=2025-02-03#7eaabe1687ea6afa84c6d7d6b8c1b3466aa56a36"
 dependencies = [
  "app_units",
  "bitflags 2.8.0",
@@ -7474,7 +7474,7 @@ dependencies = [
  "getrandom",
  "once_cell",
  "rustix",
- "windows-sys 0.59.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -7703,7 +7703,7 @@ dependencies = [
 [[package]]
 name = "to_shmem"
 version = "0.1.0"
-source = "git+https://github.com/servo/stylo?branch=2025-02-03#1977193ab231d89cf0f87ccc23c05e4ff67ee3fd"
+source = "git+https://github.com/servo/stylo?branch=2025-02-03#7eaabe1687ea6afa84c6d7d6b8c1b3466aa56a36"
 dependencies = [
  "cssparser",
  "servo_arc",
@@ -7716,7 +7716,7 @@ dependencies = [
 [[package]]
 name = "to_shmem_derive"
 version = "0.1.0"
-source = "git+https://github.com/servo/stylo?branch=2025-02-03#1977193ab231d89cf0f87ccc23c05e4ff67ee3fd"
+source = "git+https://github.com/servo/stylo?branch=2025-02-03#7eaabe1687ea6afa84c6d7d6b8c1b3466aa56a36"
 dependencies = [
  "darling",
  "proc-macro2",
@@ -8763,7 +8763,7 @@ version = "0.1.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cf221c93e13a30d793f7645a0e7762c55d169dbb0a49671918a2319d289b10bb"
 dependencies = [
- "windows-sys 0.59.0",
+ "windows-sys 0.48.0",
 ]
 
 [[package]]
diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index 3abc82e31a3..7a6936611df 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -526,10 +526,9 @@ impl Element {
         // If element’s local name is not a valid shadow host name,
         // then throw a "NotSupportedError" DOMException.
         if !is_valid_shadow_host_name(self.local_name()) {
-            match self.local_name() {
-                &local_name!("video") | &local_name!("audio")
-                    if is_ua_widget == IsUserAgentWidget::Yes => {},
-                _ => return Err(Error::NotSupported),
+            // UA shadow roots may be attached to anything
+            if is_ua_widget != IsUserAgentWidget::Yes {
+                return Err(Error::NotSupported);
             }
         }
 
@@ -3975,6 +3974,9 @@ impl SelectorsElement for DomRoot<Element> {
             NonTSPseudoClass::Indeterminate |
             NonTSPseudoClass::Invalid |
             NonTSPseudoClass::Modal |
+            NonTSPseudoClass::MozMeterOptimum |
+            NonTSPseudoClass::MozMeterSubOptimum |
+            NonTSPseudoClass::MozMeterSubSubOptimum |
             NonTSPseudoClass::Optional |
             NonTSPseudoClass::OutOfRange |
             NonTSPseudoClass::PlaceholderShown |
diff --git a/components/script/dom/htmlmeterelement.rs b/components/script/dom/htmlmeterelement.rs
index 596a792f2b5..c1d942260f8 100644
--- a/components/script/dom/htmlmeterelement.rs
+++ b/components/script/dom/htmlmeterelement.rs
@@ -2,28 +2,47 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use std::cell::Ref;
 use std::ops::{Add, Div};
 
 use dom_struct::dom_struct;
 use html5ever::{local_name, LocalName, Prefix};
 use js::rust::HandleObject;
+use style_dom::ElementState;
 
+use crate::dom::attr::Attr;
+use crate::dom::bindings::cell::DomRefCell;
 use crate::dom::bindings::codegen::Bindings::HTMLMeterElementBinding::HTMLMeterElementMethods;
+use crate::dom::bindings::codegen::Bindings::NodeBinding::Node_Binding::NodeMethods;
+use crate::dom::bindings::codegen::Bindings::ShadowRootBinding::{
+    ShadowRootMode, SlotAssignmentMode,
+};
 use crate::dom::bindings::inheritance::Castable;
 use crate::dom::bindings::num::Finite;
-use crate::dom::bindings::root::{DomRoot, MutNullableDom};
+use crate::dom::bindings::root::{Dom, DomRoot, MutNullableDom};
 use crate::dom::bindings::str::DOMString;
 use crate::dom::document::Document;
-use crate::dom::element::Element;
+use crate::dom::element::{AttributeMutation, Element};
+use crate::dom::htmldivelement::HTMLDivElement;
 use crate::dom::htmlelement::HTMLElement;
-use crate::dom::node::Node;
+use crate::dom::node::{BindContext, ChildrenMutation, Node, NodeTraits};
 use crate::dom::nodelist::NodeList;
+use crate::dom::shadowroot::IsUserAgentWidget;
+use crate::dom::virtualmethods::VirtualMethods;
 use crate::script_runtime::CanGc;
 
 #[dom_struct]
 pub(crate) struct HTMLMeterElement {
     htmlelement: HTMLElement,
     labels_node_list: MutNullableDom<NodeList>,
+    shadow_tree: DomRefCell<Option<ShadowTree>>,
+}
+
+/// Holds handles to all slots in the UA shadow tree
+#[derive(Clone, JSTraceable, MallocSizeOf)]
+#[cfg_attr(crown, crown::unrooted_must_root_lint::must_root)]
+struct ShadowTree {
+    meter_value: Dom<HTMLDivElement>,
 }
 
 /// <https://html.spec.whatwg.org/multipage/#the-meter-element>
@@ -36,6 +55,7 @@ impl HTMLMeterElement {
         HTMLMeterElement {
             htmlelement: HTMLElement::new_inherited(local_name, prefix, document),
             labels_node_list: MutNullableDom::new(None),
+            shadow_tree: Default::default(),
         }
     }
 
@@ -56,6 +76,98 @@ impl HTMLMeterElement {
             can_gc,
         )
     }
+
+    fn create_shadow_tree(&self, can_gc: CanGc) {
+        let document = self.owner_document();
+        let root = self
+            .upcast::<Element>()
+            .attach_shadow(
+                IsUserAgentWidget::Yes,
+                ShadowRootMode::Closed,
+                false,
+                SlotAssignmentMode::Manual,
+            )
+            .expect("Attaching UA shadow root failed");
+
+        let meter_value = HTMLDivElement::new(local_name!("div"), None, &document, None, can_gc);
+        root.upcast::<Node>()
+            .AppendChild(meter_value.upcast::<Node>())
+            .unwrap();
+
+        let _ = self.shadow_tree.borrow_mut().insert(ShadowTree {
+            meter_value: meter_value.as_traced(),
+        });
+        self.upcast::<Node>()
+            .dirty(crate::dom::node::NodeDamage::OtherNodeDamage);
+    }
+
+    fn shadow_tree(&self, can_gc: CanGc) -> Ref<'_, ShadowTree> {
+        if !self.upcast::<Element>().is_shadow_host() {
+            self.create_shadow_tree(can_gc);
+        }
+
+        Ref::filter_map(self.shadow_tree.borrow(), Option::as_ref)
+            .ok()
+            .expect("UA shadow tree was not created")
+    }
+
+    fn update_state(&self, can_gc: CanGc) {
+        let value = *self.Value();
+        let low = *self.Low();
+        let high = *self.High();
+        let min = *self.Min();
+        let max = *self.Max();
+        let optimum = *self.Optimum();
+
+        // If the optimum point is less than the low boundary, then the region between the minimum value and
+        // the low boundary must be treated as the optimum region, the region from the low boundary up to the
+        // high boundary must be treated as a suboptimal region, and the remaining region must be treated as
+        // an even less good region
+        let element_state = if optimum < low {
+            if value < low {
+                ElementState::OPTIMUM
+            } else if value <= high {
+                ElementState::SUB_OPTIMUM
+            } else {
+                ElementState::SUB_SUB_OPTIMUM
+            }
+        }
+        // If the optimum point is higher than the high boundary, then the situation is reversed; the region between
+        // the high boundary and the maximum value must be treated as the optimum region, the region from the high
+        // boundary down to the low boundary must be treated as a suboptimal region, and the remaining region must
+        // be treated as an even less good region.
+        else if optimum > high {
+            if value > high {
+                ElementState::OPTIMUM
+            } else if value >= low {
+                ElementState::SUB_OPTIMUM
+            } else {
+                ElementState::SUB_SUB_OPTIMUM
+            }
+        }
+        // If the optimum point is equal to the low boundary or the high boundary, or anywhere in between them,
+        // then the region between the low and high boundaries of the gauge must be treated as the optimum region,
+        // and the low and high parts, if any, must be treated as suboptimal.
+        else if (low..=high).contains(&value) {
+            ElementState::OPTIMUM
+        } else {
+            ElementState::SUB_OPTIMUM
+        };
+
+        // Set the correct pseudo class
+        self.upcast::<Element>()
+            .set_state(ElementState::METER_OPTIMUM_STATES, false);
+        self.upcast::<Element>().set_state(element_state, true);
+
+        // Update the visual width of the meter
+        let shadow_tree = self.shadow_tree(can_gc);
+        let position = (value - min) / (max - min) * 100.0;
+        let style = format!("width: {position}%");
+        shadow_tree
+            .meter_value
+            .upcast::<Element>()
+            .set_string_attribute(&local_name!("style"), style.into(), can_gc);
+    }
 }
 
 impl HTMLMeterElementMethods<crate::DomTypeHolder> for HTMLMeterElement {
@@ -210,3 +322,38 @@ impl HTMLMeterElementMethods<crate::DomTypeHolder> for HTMLMeterElement {
         );
     }
 }
+
+impl VirtualMethods for HTMLMeterElement {
+    fn super_type(&self) -> Option<&dyn VirtualMethods> {
+        Some(self.upcast::<HTMLElement>() as &dyn VirtualMethods)
+    }
+
+    fn attribute_mutated(&self, attr: &Attr, mutation: AttributeMutation) {
+        self.super_type().unwrap().attribute_mutated(attr, mutation);
+
+        let is_important_attribute = matches!(
+            attr.local_name(),
+            &local_name!("high") |
+                &local_name!("low") |
+                &local_name!("min") |
+                &local_name!("max") |
+                &local_name!("optimum") |
+                &local_name!("value")
+        );
+        if is_important_attribute {
+            self.update_state(CanGc::note());
+        }
+    }
+
+    fn children_changed(&self, mutation: &ChildrenMutation) {
+        self.super_type().unwrap().children_changed(mutation);
+
+        self.update_state(CanGc::note());
+    }
+
+    fn bind_to_tree(&self, context: &BindContext) {
+        self.super_type().unwrap().bind_to_tree(context);
+
+        self.update_state(CanGc::note());
+    }
+}
diff --git a/components/script/dom/virtualmethods.rs b/components/script/dom/virtualmethods.rs
index 6c2f8b646c6..3fb7a52899e 100644
--- a/components/script/dom/virtualmethods.rs
+++ b/components/script/dom/virtualmethods.rs
@@ -36,6 +36,7 @@ use crate::dom::htmllielement::HTMLLIElement;
 use crate::dom::htmllinkelement::HTMLLinkElement;
 use crate::dom::htmlmediaelement::HTMLMediaElement;
 use crate::dom::htmlmetaelement::HTMLMetaElement;
+use crate::dom::htmlmeterelement::HTMLMeterElement;
 use crate::dom::htmlobjectelement::HTMLObjectElement;
 use crate::dom::htmloptgroupelement::HTMLOptGroupElement;
 use crate::dom::htmloptionelement::HTMLOptionElement;
@@ -234,6 +235,9 @@ pub(crate) fn vtable_for(node: &Node) -> &dyn VirtualMethods {
         NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLMetaElement)) => {
             node.downcast::<HTMLMetaElement>().unwrap() as &dyn VirtualMethods
         },
+        NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLMeterElement)) => {
+            node.downcast::<HTMLMeterElement>().unwrap() as &dyn VirtualMethods
+        },
         NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLObjectElement)) => {
             node.downcast::<HTMLObjectElement>().unwrap() as &dyn VirtualMethods
         },
diff --git a/components/script/layout_dom/element.rs b/components/script/layout_dom/element.rs
index cdca829dca4..02862218e92 100644
--- a/components/script/layout_dom/element.rs
+++ b/components/script/layout_dom/element.rs
@@ -681,6 +681,9 @@ impl<'dom> ::selectors::Element for ServoLayoutElement<'dom> {
             NonTSPseudoClass::Indeterminate |
             NonTSPseudoClass::Invalid |
             NonTSPseudoClass::Modal |
+            NonTSPseudoClass::MozMeterOptimum |
+            NonTSPseudoClass::MozMeterSubOptimum |
+            NonTSPseudoClass::MozMeterSubSubOptimum |
             NonTSPseudoClass::Optional |
             NonTSPseudoClass::OutOfRange |
             NonTSPseudoClass::PlaceholderShown |
diff --git a/resources/servo.css b/resources/servo.css
index 5a4297fd1d6..c7e891ec58a 100644
--- a/resources/servo.css
+++ b/resources/servo.css
@@ -304,3 +304,27 @@ svg > * {
     padding: 0;
     margin: 0;
 }
+
+meter {
+  display: inline-block;
+  width: 100px;
+  height: 12px;
+  border-radius: 6px;
+  background: linear-gradient(#e6e6e6, #e6e6e6, #eeeeee 20%, #cccccc 45%, #cccccc 55%);
+  overflow: clip;
+}
+
+/* FIXME: These should use the ::-moz-meter-bar pseudo element */
+meter div {
+  height: 100%;
+}
+
+meter:-moz-meter-optimum div {
+  background: linear-gradient(#ad7, #ad7, #cea 20%, #7a3 45%, #7a3 55%);
+}
+meter:-moz-meter-sub-optimum div {
+  background: linear-gradient(#fe7, #fe7, #ffc 20%, #db3 45%, #db3 55%);
+}
+meter:-moz-meter-sub-sub-optimum div {
+  background: linear-gradient(#f77, #f77, #fcc 20%, #d44 45%, #d44 55%);
+}
\ No newline at end of file