WebVTT API: Introduction to The Web Video Text Tracks Format

Web Video Text Tracks Format (WebVTT) is a format for displaying timed text tracks (such as subtitles or captions) using the element. The primary purpose of WebVTT files is to add text overlays to a . WebVTT is a text based format, which must be encoded using . Where you can use spaces you can also use tabs. There is also a small
API available to represent and manage these tracks and the data needed
to perform the playback of the text at the correct times.

WebVTT files

The MIME type of WebVTT is

text/vtt

A WebVTT file (

.vtt

WebVTT body

The structure of a WebVTT consists of the following components, some of them optional, in this order:

• An optional byte order mark (BOM).
• The string "

  WEBVTT

  ".
• An optional text header to the right of

  WEBVTT

  .

• There must be at least one space after

  WEBVTT

  .
• You could use this to add a description to the file.
• You may use anything in the text header except newlines or the string "

  -->

  ".
  A blank line, which is equivalent to two consecutive newlines.

• Zero or more cues or comments.
• Zero or more blank lines.

Example 1 - Simplest possible WEBVTT file

WEBVTT

Example 2 - Very simple WebVTT file with a text header

WEBVTT - This file has no cues.

Example 3 - Common WebVTT example with a header and cues

WEBVTT - This file has cues.

14
00:01:14.815 --> 00:01:18.114
- What?
- Where are we now?

15
00:01:18.171 --> 00:01:20.991
- This is big bat country.

16
00:01:21.058 --> 00:01:23.868
- [ Bats Screeching ]
- They won't get in your hair. They're after the bugs.

Inner structure of a WebVTT file

Let's re-examine one of our previous examples, and look at the cue structure in a bit more detail.

WEBVTT

00:01.000 --> 00:04.000
- Never drink liquid nitrogen.

00:05.000 --> 00:09.000
- It will perforate your stomach.
- You could die.

NOTE This is the last line in the file

In the case of each cue:

• The first line is started with a time, which is the starting time for showing the text that appears underneath.
• On the same line, we then have a string of

  -->

  .
• We finish the first line with a second time, which is the ending time for showing the associated text.
• We can then have one or more lines that start with a hyphen (-), each containing part of the text track to be shown.

We can also place comments in our

.vtt

NOTE

It is important to not use "extra" blank lines within a cue, for example between the timings line and the cue payload. WebVTT is line based; a blank line will close the cue.

WebVTT comments

Comments are an optional component that can be used to add information to a WebVTT file. Comments are intended for those reading
the file and are not seen by users. Comments may contain newlines but
cannot contain a blank line, which is equivalent to two consecutive
newlines. A blank line signifies the end of a comment.

A comment cannot contain the string "

-->

&

<

&amp

&lt

&gt

>

A comment consists of three parts:

• The string

  NOTE

  .
• A space or a newline.
• Zero or more characters other than those noted above.

Example 4 - Common WebVTT example

NOTE This is a comment

Example 5 - Multi-line comment

NOTE
Another comment that is spanning
more than one line.

NOTE You can also make a comment
across more than one line this way.

Example 6 - Common comment usage

WEBVTT - Translation of that film I like

NOTE
This translation was done by Kyle so that
some friends can watch it with their parents.

1
00:02:15.000 --> 00:02:20.000
- Ta en kopp varmt te.
- Det är inte varmt.

2
00:02:20.000 --> 00:02:25.000
- Har en kopp te.
- Det smakar som te.  

NOTE This last line may not translate well.

3
00:02:25.000 --> 00:02:30.000
- Ta en kopp

Styling WebTT cues

You can style WebVTT cues by looking for elements which match the pseudo-element.

Within site CSS

video::cue {
  background-image: linear-gradient(to bottom, dimgray, lightgray);
  color: papayawhip;
}

video::cue(b) {
  color: peachpuff;
}

Here, all video elements are styled to use a gray linear gradient as their backgrounds, with a foreground color of "

papayawhip

peachpuff

The HTML snippet below actually handles displaying the media itself.

<video controls autoplay src="video.webm">
 <track default src="track.vtt">
</video>

Within the WebVTT file itself

You can also define the style directly in the WebVTT file. In this case, you insert your CSS rules into the file with each rule preceded by the string "

STYLE

WEBVTT

STYLE
::cue {
  background-image: linear-gradient(to bottom, dimgray, lightgray);
  color: papayawhip;
}
/* Style blocks cannot use blank lines nor "dash dash greater than" */

NOTE comment blocks can be used between style blocks.

STYLE
::cue(b) {
  color: peachpuff;
}

00:00:00.000 --> 00:00:10.000
- Hello <b>world</b>.

NOTE style blocks cannot appear after the first cue.

We can also use identifiers inside WebVTT file, which can be used for
defining a new style for some particular cues in the file. The example
where we wanted the transcription text to be red highlighted and the
other part to remain normal, we can define it as follows using CSS.

Where it must be noted that the CSS uses escape sequences the way they
are used in HTML pages:

WEBVTT

1
00:00.000 --> 00:02.000
That’s an, an, that’s an L!

crédit de transcription
00:04.000 --> 00:05.000
Transcrit par Célestes™

::cue(#\31) { color: lime; }
::cue(#crédit\ de\ transcription) { color: red; }

Positioning of text tracks is also supported, by including positioning
information after the timings in a cue, as seen below (see for more information):

WEBVTT

00:00:00.000 --> 00:00:04.000 position:10%,line-left align:left size:35%
Where did he go?

00:00:03.000 --> 00:00:06.500 position:90% align:right size:35%
I think he went down this lane.

00:00:04.000 --> 00:00:06.500 position:45%,line-right align:center size:35%
What are you waiting for?

WebVTT cues

A cue is a single subtitle block that has a single start time, end time, and textual payload. Example 6 consists of the header, a blank line, and then five cues separated by blank lines. A cue consists of five components:

• An optional cue identifier followed by a newline.
• Cue timings.
• Optional cue settings with at least one space before the first and between each setting.
• One or more newlines.
• The cue payload text.

Example 7 - Example of a cue

1 - Title Crawl
00:00:05.000 --> 00:00:10.000 line:0 position:20% size:60% align:start
Some time ago in a place rather distant....

Cue identifier

The identifier is a name that identifies the cue. It can be used to reference the cue from a script. It must not contain a newline and cannot contain the string "

-->

Example 8 - Cue identifier from Example 7

Title Crawl

Example 9 - Common usage of identifiers

WEBVTT

1
00:00:22.230 --> 00:00:24.606
This is the first subtitle.

2
00:00:30.739 --> 00:00:34.074
This is the second.

3
00:00:34.159 --> 00:00:35.743
Third

Cue timings

A cue timing indicates when the cue is shown. It has a start and end time which are represented by timestamps. The end time must be greater than the start time, and the start time must be greater than or equal to all previous start times. Cues may have overlapping timings.

If the WebVTT file is being used for chapters ( is

chapters

Each cue timing contains five components:

• Timestamp for start time.
• At least one space.
• The string "

  -->

  ".
• At least one space.
• Timestamp for end time.
• Which must be greater than the start time.

The timestamps must be in one of two formats:

• mm:ss.ttt

• hh:mm:ss.ttt

Where the components are defined as follows:

• hh

  is hours.
• Must be at least two digits.
• Hours can be greater than two digits (e.g., 9999:00:00.000).

• mm

  is minutes.
• Must be between 00 and 59 inclusive.

• ss

  is seconds.
• Must be between 00 and 59 inclusive.

• ttt

  is miliseconds.
• Must be between 000 and 999 inclusive.
  

Example 10 - Basic cue timing examples

00:00:22.230 --> 00:00:24.606
00:00:30.739 --> 00:00:34.074
00:00:34.159 --> 00:00:35.743
00:00:35.827 --> 00:00:40.122

Example 11 - Overlapping cue timing examples

00:00:00.000 --> 00:00:10.000
00:00:05.000 --> 00:01:00.000
00:00:30.000 --> 00:00:50.000

Example 12 - Non-overlapping cue timing examples

00:00:00.000 --> 00:00:10.000
00:00:10.000 --> 00:01:00.581
00:01:00.581 --> 00:02:00.100
00:02:01.000 --> 00:02:01.000

Cue settings

Cue settings are optional components used to position where the cue
payload text will be displayed over the video. This includes whether the
text is displayed horizontally or vertically. There can be zero or more
of them, and they can be used in any order so long as each setting is
used no more than once.

The cue settings are added to the right of the cue timings. There must be one or more spaces between the cue timing and the first setting and between each setting. A setting's name and value are separated by a colon. The settings are case sensitive so use lower case as shown. There are five cue settings:

• vertical
• Indicates that the text will be displayed vertically rather than horizontally, such as in some Asian languages.

• line    
• Specifies where text appears vertically. If vertical is set, line specifies where text appears horizontally.
• Value can be a line number.        
• The line height is the height of the first line of the cue as it appears on the video.
• Positive numbers indicate top down.
• Negative numbers indicate bottom up.
• Or value can be a percentage.        
• Must be an integer (i.e., no decimals) between 0 and 100 inclusive.
• Must be followed by a percent sign (%).

• position
• Specifies where the text will appear horizontally. If vertical is set, position specifies where the text will appear vertically.
• Value is a percentage.
• Must be an integer (no decimals) between 0 and 100 inclusive.
• Must be followed by a percent sign (%).

• size
• Specifies the width of the text area. If vertical is set, size specifies the height of the text area.
• Value is a percentage.
• Must be an integer (i.e., no decimals) between 0 and 100 inclusive.
• Must be followed by a percent sign (%).

• align

• Specifies the alignment of the text. Text is aligned within the space given by the size cue setting if it is set.

Example 13 - Cue setting examples

The first line demonstrates no settings. The second line might be used to overlay text on a sign or label. The third line might be used for a title. The last line might be used for an Asian language.

00:00:05.000 --> 00:00:10.000
00:00:05.000 --> 00:00:10.000 line:63% position:72% align:start
00:00:05.000 --> 00:00:10.000 line:0 position:20% size:60% align:start
00:00:05.000 --> 00:00:10.000 vertical:rt line:-1 align:end

Cue payload

The payload is where the main information or content is located. In
normal usage the payload contains the subtitles to be displayed. The
payload text may contain newlines but it cannot contain a blank line,
which is equivalent to two consecutive newlines. A blank line signifies
the end of a cue.

A cue text payload cannot contain the string "

-->

&

<

&

<

>

>

In addition to the three escape sequences mentioned above, there are fours others. They are listed in the table below.

Cue payload text tags

There are a number of tags, such as

<bold>

• Timestamp tag
• The timestamp must be greater that the cue's start timestamp, greater than any previous timestamp in the cue payload, and less than the cue's end timestamp. The active text is the text between the timestamp and the next timestamp or to the end of the payload if there is not another timestamp in the payload. Any text before the active text in the payload is previous text . Any text beyond the active text is future text . This enables karaoke style captions.

Example 12 - Karaoke style text

1
00:16.500 --> 00:18.500
When the moon <00:17.500>hits your eye

1
00:00:18.500 --> 00:00:20.500
Like a <00:19.000>big-a <00:19.500>pizza <00:20.000>pie

1
00:00:20.500 --> 00:00:21.500
That's <00:00:21.000>amore

The following tags are the HTML tags allowed in a cue and require opening and closing tags (e.g., 

<b>text</b>

• Class tag (

  <c></c>

  )
• Style the contained text using a CSS class.

Example 14 - Class tag

<c.classname>text</c>

• Italics tag (

  <i></i>

  )
• Italicize the contained text.

Example 15 - Italics tag

<i>text</i>

• Bold tag (

  <b></b>

  )
• Bold the contained text.

Example 16 - Bold tag

<b>text</b>

• Underline tag (

  <u></u>

  )
• Underline the contained text.

Example 17 - Underline tag

<u>text</u>

• Ruby tag (

  <ruby></ruby>

  )
• Used with ruby text tags to display (i.e., small annotative characters above other characters).

Example 18 - Ruby tag

<ruby>WWW<rt>World Wide Web</rt>oui<rt>yes</rt></ruby>

• Ruby text tag (

  <rt></rt>

  )
• Used with ruby tags to display (i.e., small annotative characters above other characters).

Example 19 - Ruby text tag

<ruby>WWW<rt>World Wide Web</rt>oui<rt>yes</rt></ruby>

• Voice tag (

  <v></v>

  )
• Similar to class tag, also used to style the contained text using CSS.

Example 20 - Voice tag

<v Bob>text</v>

Interfaces

There are two interfaces or APIs used in WebVTT which are:

VTTCue interface

It is used for providing an interface in Document Object Model API, where different attributes supported by it can be used to prepare and alter the cues in number of ways.

Constructor is the first point for starting the Cue which is defined using the default constructor VTTCue(startTime, endTime, text) where starting time, ending time and text for cue can be adjusted. After that we can set the region for that particular cue to which this cue belongs using cue.region. Vertical, horizontal, line, lineAlign, Position, positionAlign, text, size and Align can be used to alter the cue and its formation, just like we can alter the objects form, shape and visibility in HTML using CSS.

But the VTTCue interface is within the WebVTT provides the vast range of adjustment variables which can be used directly to alter the Cue. Following interface can be used to expose WebVTT cues in DOM API:

enum AutoKeyword { "auto" };
enum DirectionSetting { "" /* horizontal */, "rl", "lr" };
enum LineAlignSetting { "start", "center", "end" };
enum PositionAlignSetting { "line-left", "center", "line-right", "auto" };
enum AlignSetting { "start", "center", "end", "left", "right" };
[Constructor(double startTime, double endTime, DOMString text)]
interface VTTCue : TextTrackCue {
  attribute VTTRegion? region;
  attribute DirectionSetting vertical;
  attribute boolean snapToLines;
  attribute (double or AutoKeyword) line;
  attribute LineAlignSetting lineAlign;
  attribute (double or AutoKeyword) position;
  attribute PositionAlignSetting positionAlign;
  attribute double size;
  attribute AlignSetting align;
  attribute DOMString text;
  DocumentFragment getCueAsHTML();
};

VTT Region interface

This is the second interface in WebVTT API.

The new keyword can be used for defining a new VTTRegion object which can then be used for containing the multiple cues. There are several properties of VTTRegion which are width, lines, regionAnchorX,
RegionAnchorY, viewportAnchorX, viewportAnchorY and scroll that can be used to specify the look and feel of this VTT region.

The interface code is given below which can be used to expose the WebVTT regions in DOM API:

enum ScrollSetting { "" /* none */, "up" };
[Constructor]
interface VTTRegion {
  attribute double width;
  attribute long lines;
  attribute double regionAnchorX;
  attribute double regionAnchorY;
  attribute double viewportAnchorX;
  attribute double viewportAnchorY;
  attribute ScrollSetting scroll;
};

Methods and properties

The methods used in WebVTT are those which are used to alter the cue or region as the attributes for both interfaces are different. We can categorize them for better understanding relating to each interface in WebVTT:

• VTTCue
• The methods which are available in this interface are:
• 
  GetCueAsHTML to get the HTML of that Cue.
• VTT Constructor for creating new objects of Cues.
• Autokeyword.
• DirectionSetting: to set the direction of caption or text in a file.
• LineAlignment: to adjust the line alignment.
• PositionAlignSetting: to adjust the position of text.
• VTTRegion
• The methods used for region are listed below along with description of their functionality:
• ScrollSetting: For adjusting the scrolling setting of all nodes present in given region.
• VTT Region Constructor: for construction of new VTT Regions.

Tutorial on how to write a WebVTT file

There are few steps that can be followed to write a simple webVTT
file. Before start, it must be noted that you can make use of a notepad
and then save the file as ‘.vtt’ file. Steps are given below:

1. Open a notepad.

2. The first line of WebVTT is standardized similar in the way some other languages require you to put headers as the file starts to indicate the file type. One the very first line you have to write.

WEBVTT

3. Leave the second line blank, and on the third line the time for first cue is to be specified. For example, for a first cue starting at time 1 second and ending at 5 seconds, it is written as:

00:01.000 --> 00:05.000

1. On the next line you can write the caption for this cue which will run from 1st second to the 5th second, inclusive.
2. Following the similar steps, a complete WebVTT file for specific video or audio file can be made.

CSS pseudo-classes

CSS pseudo classes allow us to classify the type of object which we want to differentiate from other types of objects. It works in similar manner in WebVTT files as it works in HTML file.

It is one of the good features supported by WebVTT is the localization and use of class elements which can be used in same way they are used in HTML and CSS to classify the style for particular type of objects, but here these are used for styling and classifying the Cues as shown below:

WEBVTT

04:02.500 --> 04:05.000
J’ai commencé le basket à l'âge de 13, 14 ans

04:05.001 --> 04:07.800
Sur les <i.foreignphrase><lang en>playground</lang></i>, ici à Montpellier

In the above example it can be observed that we can use the identifier and pseudo class name for defining the language of caption, where

<i>

The type of pseudo class is determined by the selector it is using and working is similar in nature as it works in HTML. Following CSS pseudo classes can be used:

• Lang (Lanugage): e.g., p:lang(it).
• Link: e.g., a:link.
• Nth-last-child: e.g., p:nth-last-child(2).
• Nth-child(n): e.g., p:nth-child(2).

Where p and a are the tags which are used in HTML for paragraph and link, respectively and they can be replaced by identifiers which are used for Cues in WebVTT file.

Specifications

Browser compatibility

VTTCue

TextTrack

Notes

Prior to Firefox 50, the

AlignSetting

true

false

Prior to Firefox 58, the

REGION

media.webvtt.regions.enabled

true

Credits

• Source:
• Published under Open CC Attribution ShareAlike 3.0 licence