script: Clean up attribute access a little bit in Element (#43064)

- Use modern Rust conveniences such as `unwrap_or_default`
 - Unabbreviate `attr`
- Unify the lowercase ASCII name assertion and make it a debug assertion
 - Use `unreachable!` instead of panic
- Expose two attribute getters that follow the behavior of two
specification concepts and what we expect internally in Servo:
- One that takes a namespace, but does not require lowercase attribute
names. ([specification
concept](https://dom.spec.whatwg.org/#concept-element-attributes-get-by-namespace>))
- One that does not take a namespace, but does require lowercase
attribute names. ([specification
concept](https://dom.spec.whatwg.org/#concept-element-attributes-get-by-name))
Testing: This should not really change behavior so should be covered by
existing tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/html/htmlmediaelement.rs

@@ -1092,9 +1092,7 @@ impl HTMLMediaElement {
         let mode = if self.src_object.borrow().is_some() {
             // If the media element has an assigned media provider object, then let mode be object.
             Mode::Object
-        } else if let Some(attribute) = self
-            .upcast::<Element>()
-            .get_attribute(&ns!(), &local_name!("src"))
+        } else if let Some(attribute) = self.upcast::<Element>().get_attribute(&local_name!("src"))
         {
             // Otherwise, if the media element has no assigned media provider object but has a src
             // attribute, then let mode be attribute.
@@ -1213,7 +1211,7 @@ impl HTMLMediaElement {
         // its src attribute's value is the empty string, then end the synchronous section, and jump
         // down to the failed with elements step below.
         let Some(src) = element
-            .get_attribute(&ns!(), &local_name!("src"))
+            .get_attribute(&local_name!("src"))
             .filter(|attribute| !attribute.value().is_empty())
         else {
             self.load_from_source_child_failure_steps(source);
@@ -1223,7 +1221,7 @@ impl HTMLMediaElement {
         // Step 9.children.3. If candidate has a media attribute whose value does not match the
         // environment, then end the synchronous section, and jump down to the failed with elements
         // step below.
-        if let Some(media) = element.get_attribute(&ns!(), &local_name!("media")) {
+        if let Some(media) = element.get_attribute(&local_name!("media")) {
             if !MediaList::matches_environment(&element.owner_document(), &media.value()) {
                 self.load_from_source_child_failure_steps(source);
                 return;
@@ -1244,7 +1242,7 @@ impl HTMLMediaElement {
         // type (including any codecs described by the codecs parameter, for types that define that
         // parameter), represents a type that the user agent knows it cannot render, then end the
         // synchronous section, and jump down to the failed with elements step below.
-        if let Some(type_) = element.get_attribute(&ns!(), &local_name!("type")) {
+        if let Some(type_) = element.get_attribute(&local_name!("type")) {
             if ServoMedia::get().can_play_type(&type_.value()) == SupportsMediaType::No {
                 self.load_from_source_child_failure_steps(source);
                 return;