script: Create structures for TextTrack API

Fill out the basics for the WebIDLs for the following:

  - TextTrack
  - TextTrackCue
  - TextTrackCueList
  - TextTrackList

components/atoms/static_atoms.txt

@@ -1,6 +1,7 @@
 DOMContentLoaded
 abort
 activate
+addtrack
 beforeunload
 button
 canplay
@@ -69,6 +70,7 @@ range
 readystatechange
 reftest-wait
 rejectionhandled
+removetrack
 reset
 resize
 right

components/script/dom/htmlmediaelement.rs

@@ -12,6 +12,7 @@ use crate::dom::bindings::codegen::Bindings::HTMLMediaElementBinding::HTMLMediaE
 use crate::dom::bindings::codegen::Bindings::HTMLSourceElementBinding::HTMLSourceElementMethods;
 use crate::dom::bindings::codegen::Bindings::MediaErrorBinding::MediaErrorConstants::*;
 use crate::dom::bindings::codegen::Bindings::MediaErrorBinding::MediaErrorMethods;
+use crate::dom::bindings::codegen::Bindings::TextTrackBinding::{TextTrackKind, TextTrackMode};
 use crate::dom::bindings::codegen::InheritTypes::{ElementTypeId, HTMLElementTypeId};
 use crate::dom::bindings::codegen::InheritTypes::{HTMLMediaElementTypeId, NodeTypeId};
 use crate::dom::bindings::error::{Error, ErrorResult};
@@ -33,6 +34,8 @@ use crate::dom::mediaerror::MediaError;
 use crate::dom::node::{document_from_node, window_from_node, Node, NodeDamage, UnbindContext};
 use crate::dom::performanceresourcetiming::InitiatorType;
 use crate::dom::promise::Promise;
+use crate::dom::texttrack::TextTrack;
+use crate::dom::texttracklist::TextTrackList;
 use crate::dom::timeranges::{TimeRanges, TimeRangesContainer};
 use crate::dom::virtualmethods::VirtualMethods;
 use crate::fetch::FetchCanceller;
@@ -191,6 +194,8 @@ pub struct HTMLMediaElement {
     /// https://html.spec.whatwg.org/multipage/#dom-media-played
     #[ignore_malloc_size_of = "Rc"]
     played: Rc<DomRefCell<TimeRangesContainer>>,
+    /// https://html.spec.whatwg.org/multipage/#dom-media-texttracks
+    text_tracks_list: MutNullableDom<TextTrackList>,
 }
 
 /// <https://html.spec.whatwg.org/multipage/#dom-media-networkstate>
@@ -243,6 +248,7 @@ impl HTMLMediaElement {
             seeking: Cell::new(false),
             resource_url: DomRefCell::new(None),
             played: Rc::new(DomRefCell::new(TimeRangesContainer::new())),
+            text_tracks_list: Default::default(),
         }
     }
 
@@ -1376,6 +1382,37 @@ impl HTMLMediaElementMethods for HTMLMediaElement {
     fn Played(&self) -> DomRoot<TimeRanges> {
         TimeRanges::new(self.global().as_window(), self.played.clone())
     }
+
+    // https://html.spec.whatwg.org/multipage/#dom-media-texttracks
+    fn TextTracks(&self) -> DomRoot<TextTrackList> {
+        let window = window_from_node(self);
+        self.text_tracks_list
+            .or_init(|| TextTrackList::new(&window, &[]))
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-media-addtexttrack
+    fn AddTextTrack(
+        &self,
+        kind: TextTrackKind,
+        label: DOMString,
+        language: DOMString,
+    ) -> DomRoot<TextTrack> {
+        let window = window_from_node(self);
+        // Step 1 & 2
+        // FIXME(#22314, dlrobertson) set the ready state to Loaded
+        let track = TextTrack::new(
+            &window,
+            "".into(),
+            kind,
+            label,
+            language,
+            TextTrackMode::Hidden,
+        );
+        // Step 3 & 4
+        self.TextTracks().add(&track);
+        // Step 5
+        DomRoot::from_ref(&track)
+    }
 }
 
 impl VirtualMethods for HTMLMediaElement {

components/script/dom/mod.rs

@@ -460,6 +460,10 @@ pub mod text;
 pub mod textcontrol;
 pub mod textdecoder;
 pub mod textencoder;
+pub mod texttrack;
+pub mod texttrackcue;
+pub mod texttrackcuelist;
+pub mod texttracklist;
 pub mod timeranges;
 pub mod touch;
 pub mod touchevent;

components/script/dom/texttrack.rs

@@ -0,0 +1,145 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * 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 crate::dom::bindings::codegen::Bindings::TextTrackBinding::{
+    self, TextTrackKind, TextTrackMethods, TextTrackMode,
+};
+use crate::dom::bindings::error::{Error, ErrorResult};
+use crate::dom::bindings::reflector::{reflect_dom_object, DomObject};
+use crate::dom::bindings::root::{DomRoot, MutNullableDom};
+use crate::dom::bindings::str::DOMString;
+use crate::dom::eventtarget::EventTarget;
+use crate::dom::texttrackcue::TextTrackCue;
+use crate::dom::texttrackcuelist::TextTrackCueList;
+use crate::dom::window::Window;
+use dom_struct::dom_struct;
+use std::cell::Cell;
+
+#[dom_struct]
+pub struct TextTrack {
+    eventtarget: EventTarget,
+    kind: TextTrackKind,
+    label: String,
+    language: String,
+    id: String,
+    mode: Cell<TextTrackMode>,
+    cue_list: MutNullableDom<TextTrackCueList>,
+}
+
+impl TextTrack {
+    pub fn new_inherited(
+        id: DOMString,
+        kind: TextTrackKind,
+        label: DOMString,
+        language: DOMString,
+        mode: TextTrackMode,
+    ) -> TextTrack {
+        TextTrack {
+            eventtarget: EventTarget::new_inherited(),
+            kind: kind,
+            label: label.into(),
+            language: language.into(),
+            id: id.into(),
+            mode: Cell::new(mode),
+            cue_list: Default::default(),
+        }
+    }
+
+    pub fn new(
+        window: &Window,
+        id: DOMString,
+        kind: TextTrackKind,
+        label: DOMString,
+        language: DOMString,
+        mode: TextTrackMode,
+    ) -> DomRoot<TextTrack> {
+        reflect_dom_object(
+            Box::new(TextTrack::new_inherited(id, kind, label, language, mode)),
+            window,
+            TextTrackBinding::Wrap,
+        )
+    }
+
+    pub fn get_cues(&self) -> DomRoot<TextTrackCueList> {
+        self.cue_list
+            .or_init(|| TextTrackCueList::new(&self.global().as_window(), &[]))
+    }
+
+    pub fn id(&self) -> &str {
+        &self.id
+    }
+}
+
+impl TextTrackMethods for TextTrack {
+    // https://html.spec.whatwg.org/multipage/#dom-texttrack-kind
+    fn Kind(&self) -> TextTrackKind {
+        self.kind
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrack-label
+    fn Label(&self) -> DOMString {
+        DOMString::from(self.label.clone())
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrack-language
+    fn Language(&self) -> DOMString {
+        DOMString::from(self.language.clone())
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrack-id
+    fn Id(&self) -> DOMString {
+        DOMString::from(self.id.clone())
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrack-mode
+    fn Mode(&self) -> TextTrackMode {
+        self.mode.get()
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrack-mode
+    fn SetMode(&self, value: TextTrackMode) {
+        self.mode.set(value)
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrack-cues
+    fn GetCues(&self) -> Option<DomRoot<TextTrackCueList>> {
+        match self.Mode() {
+            TextTrackMode::Disabled => None,
+            _ => Some(self.get_cues()),
+        }
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrack-addcue
+    fn AddCue(&self, cue: &TextTrackCue) -> ErrorResult {
+        // FIXME(#22314, dlrobertson) add Step 1 & 2
+        // Step 3
+        if let Some(old_track) = cue.get_track() {
+            // gecko calls RemoveCue when the given cue
+            // has an associated track, but doesn't return
+            // the error from it, so we wont either.
+            if let Err(_) = old_track.RemoveCue(cue) {
+                warn!("Failed to remove cues for the added cue's text track");
+            }
+        }
+        // Step 4
+        self.get_cues().add(cue);
+        Ok(())
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrack-removecue
+    fn RemoveCue(&self, cue: &TextTrackCue) -> ErrorResult {
+        // Step 1
+        let cues = self.get_cues();
+        let index = match cues.find(cue) {
+            Some(i) => Ok(i),
+            None => Err(Error::NotFound),
+        }?;
+        // Step 2
+        cues.remove(index);
+        Ok(())
+    }
+
+    // https://html.spec.whatwg.org/multipage/#handler-texttrack-oncuechange
+    event_handler!(oncuechange, GetOncuechange, SetOncuechange);
+}

components/script/dom/texttrackcue.rs

@@ -0,0 +1,111 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * 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 crate::dom::bindings::cell::DomRefCell;
+use crate::dom::bindings::codegen::Bindings::TextTrackCueBinding::{self, TextTrackCueMethods};
+use crate::dom::bindings::num::Finite;
+use crate::dom::bindings::reflector::reflect_dom_object;
+use crate::dom::bindings::root::{Dom, DomRoot};
+use crate::dom::bindings::str::DOMString;
+use crate::dom::eventtarget::EventTarget;
+use crate::dom::texttrack::TextTrack;
+use crate::dom::window::Window;
+use dom_struct::dom_struct;
+use std::cell::Cell;
+
+#[dom_struct]
+pub struct TextTrackCue {
+    eventtarget: EventTarget,
+    id: DomRefCell<DOMString>,
+    track: Option<Dom<TextTrack>>,
+    start_time: Cell<f64>,
+    end_time: Cell<f64>,
+    pause_on_exit: Cell<bool>,
+}
+
+impl TextTrackCue {
+    // FIXME(#22314, dlrobertson) implement VTTCue.
+    #[allow(dead_code)]
+    pub fn new_inherited(id: DOMString, track: Option<&TextTrack>) -> TextTrackCue {
+        TextTrackCue {
+            eventtarget: EventTarget::new_inherited(),
+            id: DomRefCell::new(id),
+            track: track.map(Dom::from_ref),
+            start_time: Cell::new(0.),
+            end_time: Cell::new(0.),
+            pause_on_exit: Cell::new(false),
+        }
+    }
+
+    // FIXME(#22314, dlrobertson) implement VTTCue.
+    #[allow(dead_code)]
+    pub fn new(window: &Window, id: DOMString, track: Option<&TextTrack>) -> DomRoot<TextTrackCue> {
+        reflect_dom_object(
+            Box::new(TextTrackCue::new_inherited(id, track)),
+            window,
+            TextTrackCueBinding::Wrap,
+        )
+    }
+
+    pub fn id(&self) -> DOMString {
+        self.id.borrow().clone()
+    }
+
+    pub fn get_track(&self) -> Option<DomRoot<TextTrack>> {
+        self.track.as_ref().map(|t| DomRoot::from_ref(&**t))
+    }
+}
+
+impl TextTrackCueMethods for TextTrackCue {
+    // https://html.spec.whatwg.org/multipage/#dom-texttrackcue-id
+    fn Id(&self) -> DOMString {
+        self.id()
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrackcue-id
+    fn SetId(&self, value: DOMString) {
+        *self.id.borrow_mut() = value;
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrackcue-track
+    fn GetTrack(&self) -> Option<DomRoot<TextTrack>> {
+        self.get_track()
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrackcue-starttime
+    fn StartTime(&self) -> Finite<f64> {
+        Finite::wrap(self.start_time.get())
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrackcue-starttime
+    fn SetStartTime(&self, value: Finite<f64>) {
+        self.start_time.set(*value);
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrackcue-endtime
+    fn EndTime(&self) -> Finite<f64> {
+        Finite::wrap(self.end_time.get())
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrackcue-endtime
+    fn SetEndTime(&self, value: Finite<f64>) {
+        self.end_time.set(*value);
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrackcue-pauseonexit
+    fn PauseOnExit(&self) -> bool {
+        self.pause_on_exit.get()
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrackcue-pauseonexit
+    fn SetPauseOnExit(&self, value: bool) {
+        self.pause_on_exit.set(value);
+    }
+
+    // https://html.spec.whatwg.org/multipage/#handler-texttrackcue-onenter
+    event_handler!(onenter, GetOnenter, SetOnenter);
+
+    // https://html.spec.whatwg.org/multipage/#handler-texttrackcue-onexit
+    event_handler!(onexit, GetOnexit, SetOnexit);
+}

components/script/dom/texttrackcuelist.rs

@@ -0,0 +1,91 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * 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 crate::dom::bindings::cell::DomRefCell;
+use crate::dom::bindings::codegen::Bindings::TextTrackCueListBinding::{
+    self, TextTrackCueListMethods,
+};
+use crate::dom::bindings::reflector::{reflect_dom_object, Reflector};
+use crate::dom::bindings::root::{Dom, DomRoot};
+use crate::dom::bindings::str::DOMString;
+use crate::dom::texttrackcue::TextTrackCue;
+use crate::dom::window::Window;
+use dom_struct::dom_struct;
+
+#[dom_struct]
+pub struct TextTrackCueList {
+    reflector_: Reflector,
+    dom_cues: DomRefCell<Vec<Dom<TextTrackCue>>>,
+}
+
+impl TextTrackCueList {
+    pub fn new_inherited(cues: &[&TextTrackCue]) -> TextTrackCueList {
+        TextTrackCueList {
+            reflector_: Reflector::new(),
+            dom_cues: DomRefCell::new(cues.iter().map(|g| Dom::from_ref(&**g)).collect()),
+        }
+    }
+
+    pub fn new(window: &Window, cues: &[&TextTrackCue]) -> DomRoot<TextTrackCueList> {
+        reflect_dom_object(
+            Box::new(TextTrackCueList::new_inherited(cues)),
+            window,
+            TextTrackCueListBinding::Wrap,
+        )
+    }
+
+    pub fn item(&self, idx: usize) -> Option<DomRoot<TextTrackCue>> {
+        self.dom_cues
+            .borrow()
+            .get(idx)
+            .map(|t| DomRoot::from_ref(&**t))
+    }
+
+    pub fn find(&self, cue: &TextTrackCue) -> Option<usize> {
+        self.dom_cues
+            .borrow()
+            .iter()
+            .enumerate()
+            .filter(|(_, c)| **c == cue)
+            .next()
+            .map(|(i, _)| i)
+    }
+
+    pub fn add(&self, cue: &TextTrackCue) {
+        // Only add a cue if it does not exist in the list
+        if self.find(cue).is_none() {
+            self.dom_cues.borrow_mut().push(Dom::from_ref(cue));
+        }
+    }
+
+    pub fn remove(&self, idx: usize) {
+        self.dom_cues.borrow_mut().remove(idx);
+    }
+}
+
+impl TextTrackCueListMethods for TextTrackCueList {
+    // https://html.spec.whatwg.org/multipage/#dom-texttrackcuelist-length
+    fn Length(&self) -> u32 {
+        self.dom_cues.borrow().len() as u32
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrackcuelist-item
+    fn IndexedGetter(&self, idx: u32) -> Option<DomRoot<TextTrackCue>> {
+        self.item(idx as usize)
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttrackcuelist-getcuebyid
+    fn GetCueById(&self, id: DOMString) -> Option<DomRoot<TextTrackCue>> {
+        if id.is_empty() {
+            None
+        } else {
+            self.dom_cues
+                .borrow()
+                .iter()
+                .filter(|cue| cue.id() == id)
+                .next()
+                .map(|t| DomRoot::from_ref(&**t))
+        }
+    }
+}

components/script/dom/texttracklist.rs

@@ -0,0 +1,103 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * 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 crate::dom::bindings::cell::DomRefCell;
+use crate::dom::bindings::codegen::Bindings::TextTrackListBinding::{self, TextTrackListMethods};
+use crate::dom::bindings::inheritance::Castable;
+use crate::dom::bindings::reflector::reflect_dom_object;
+use crate::dom::bindings::root::{Dom, DomRoot};
+use crate::dom::bindings::str::DOMString;
+use crate::dom::eventtarget::EventTarget;
+use crate::dom::texttrack::TextTrack;
+use crate::dom::window::Window;
+use dom_struct::dom_struct;
+
+#[dom_struct]
+pub struct TextTrackList {
+    eventtarget: EventTarget,
+    dom_tracks: DomRefCell<Vec<Dom<TextTrack>>>,
+}
+
+impl TextTrackList {
+    pub fn new_inherited(tracks: &[&TextTrack]) -> TextTrackList {
+        TextTrackList {
+            eventtarget: EventTarget::new_inherited(),
+            dom_tracks: DomRefCell::new(tracks.iter().map(|g| Dom::from_ref(&**g)).collect()),
+        }
+    }
+
+    pub fn new(window: &Window, tracks: &[&TextTrack]) -> DomRoot<TextTrackList> {
+        reflect_dom_object(
+            Box::new(TextTrackList::new_inherited(tracks)),
+            window,
+            TextTrackListBinding::Wrap,
+        )
+    }
+
+    pub fn item(&self, idx: usize) -> Option<DomRoot<TextTrack>> {
+        self.dom_tracks
+            .borrow()
+            .get(idx as usize)
+            .map(|t| DomRoot::from_ref(&**t))
+    }
+
+    pub fn find(&self, track: &TextTrack) -> Option<usize> {
+        self.dom_tracks
+            .borrow()
+            .iter()
+            .enumerate()
+            .filter(|(_, t)| **t == track)
+            .next()
+            .map(|(i, _)| i)
+    }
+
+    pub fn add(&self, track: &TextTrack) {
+        // Only add a track if it does not exist in the list
+        if self.find(track).is_none() {
+            self.dom_tracks.borrow_mut().push(Dom::from_ref(track));
+        };
+        self.upcast::<EventTarget>().fire_event(atom!("addtrack"));
+    }
+
+    // FIXME(#22314, dlrobertson) allow TextTracks to be
+    // removed from the TextTrackList.
+    #[allow(dead_code)]
+    pub fn remove(&self, idx: usize) {
+        self.dom_tracks.borrow_mut().remove(idx);
+        self.upcast::<EventTarget>()
+            .fire_event(atom!("removetrack"));
+    }
+}
+
+impl TextTrackListMethods for TextTrackList {
+    // https://html.spec.whatwg.org/multipage/#dom-texttracklist-length
+    fn Length(&self) -> u32 {
+        self.dom_tracks.borrow().len() as u32
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttracklist-item
+    fn IndexedGetter(&self, idx: u32) -> Option<DomRoot<TextTrack>> {
+        self.item(idx as usize)
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-texttracklist-gettrackbyid
+    fn GetTrackById(&self, id: DOMString) -> Option<DomRoot<TextTrack>> {
+        let id_str = String::from(id.clone());
+        self.dom_tracks
+            .borrow()
+            .iter()
+            .filter(|track| track.id() == &id_str)
+            .next()
+            .map(|t| DomRoot::from_ref(&**t))
+    }
+
+    // https://html.spec.whatwg.org/multipage/#handler-texttracklist-onchange
+    event_handler!(onchange, GetOnchange, SetOnchange);
+
+    // https://html.spec.whatwg.org/multipage/#handler-texttracklist-onaddtrack
+    event_handler!(onaddtrack, GetOnaddtrack, SetOnaddtrack);
+
+    // https://html.spec.whatwg.org/multipage/#handler-texttracklist-onremovetrack
+    event_handler!(onremovetrack, GetOnremovetrack, SetOnremovetrack);
+}

components/script/dom/webidls/HTMLMediaElement.webidl

@@ -61,6 +61,6 @@ interface HTMLMediaElement : HTMLElement {
   // tracks
   // readonly attribute AudioTrackList audioTracks;
   // readonly attribute VideoTrackList videoTracks;
-  // readonly attribute TextTrackList textTracks;
-  // TextTrack addTextTrack(TextTrackKind kind, optional DOMString label = "", optional DOMString language = "");
+  readonly attribute TextTrackList textTracks;
+  TextTrack addTextTrack(TextTrackKind kind, optional DOMString label = "", optional DOMString language = "");
 };

components/script/dom/webidls/TextTrack.webidl

@@ -0,0 +1,30 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * 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/. */
+
+// https://html.spec.whatwg.org/multipage/#texttrack
+
+enum TextTrackMode { "disabled",  "hidden",  "showing" };
+enum TextTrackKind { "subtitles",  "captions",  "descriptions",  "chapters",  "metadata" };
+
+[Exposed=Window]
+interface TextTrack : EventTarget {
+  readonly attribute TextTrackKind kind;
+  readonly attribute DOMString label;
+  readonly attribute DOMString language;
+
+  readonly attribute DOMString id;
+  // readonly attribute DOMString inBandMetadataTrackDispatchType;
+
+  attribute TextTrackMode mode;
+
+  readonly attribute TextTrackCueList? cues;
+  // readonly attribute TextTrackCueList? activeCues;
+
+  [Throws]
+  void addCue(TextTrackCue cue);
+  [Throws]
+  void removeCue(TextTrackCue cue);
+
+  attribute EventHandler oncuechange;
+};

components/script/dom/webidls/TextTrackCue.webidl

@@ -0,0 +1,18 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * 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/. */
+
+// https://html.spec.whatwg.org/multipage/#texttrackcue
+
+[Exposed=Window]
+interface TextTrackCue : EventTarget {
+  readonly attribute TextTrack? track;
+
+  attribute DOMString id;
+  attribute double startTime;
+  attribute double endTime;
+  attribute boolean pauseOnExit;
+
+  attribute EventHandler onenter;
+  attribute EventHandler onexit;
+};

components/script/dom/webidls/TextTrackCueList.webidl

@@ -0,0 +1,12 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * 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/. */
+
+// https://html.spec.whatwg.org/multipage/#texttrackcuelist
+
+[Exposed=Window]
+interface TextTrackCueList {
+  readonly attribute unsigned long length;
+  getter TextTrackCue (unsigned long index);
+  TextTrackCue? getCueById(DOMString id);
+};

components/script/dom/webidls/TextTrackList.webidl

@@ -0,0 +1,16 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * 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/. */
+
+// https://html.spec.whatwg.org/multipage/#texttracklist
+
+[Exposed=Window]
+interface TextTrackList : EventTarget {
+  readonly attribute unsigned long length;
+  getter TextTrack (unsigned long index);
+  TextTrack? getTrackById(DOMString id);
+
+  attribute EventHandler onchange;
+  attribute EventHandler onaddtrack;
+  attribute EventHandler onremovetrack;
+};