Web Video Text Tracks Format (WebVTT) is a format for displaying timed text tracks (such as subtitles or captions) using the 
<track>
 element. The primary purpose of WebVTT files is to add text overlays to a 
<video>
. WebVTT is a text based format, which must be encoded using UTF-8. 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
) contains cues, which can be either a single line or multiple lines, as shown below:

WebVTT body

The structure of a WebVTT consists of the following components, some of them optional, in this order:
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:
We can also place comments in our 
.vtt
file, to help us remember important information about the parts of our file. These should be on separate lines, starting with the string 
NOTE
. You'll find more about these in the below section.
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 "
-->
", the ampersand character (
&
), or the less-than sign (
<
). If you wish to use such characters, you need to escape them using for example 
&amp
; for ampersand and 
&lt
; for less-than. It is also recommended that you use the greater-than escape sequence (
&gt
;) instead of the greater-than character (
>
) to avoid confusion with tags.
A comment consists of three parts:
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 
::cue
 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
". In addition, text boldfaced using the 
<b>
 element are colored "
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
" all by itelf on a line of text, as shown below:
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 Cue settings 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:
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 "
-->
". It must end with a single newline. They do not have to be unique, although it is common to number them (e.g., 1, 2, 3, ...).
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 (
<track>
 
kind
 is 
chapters
) then the file cannot have overlapping timings.
Each cue timing contains five components:
The timestamps must be in one of two formats:
Where the components are defined as follows:
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:
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 "
-->
", the ampersand character (
&
), or the less-than sign (
<
). Instead use the escape sequence "
&amp;
" for ampersand and "
&lt;
" for less-than. It is also recommended that you use the greater-than escape sequence "
&gt;
" instead of the greater-than character (
>
) to avoid confusion with tags. If you are using the WebVTT file for metadata these restrictions do not apply.
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>
, that can be used. However, if the WebVTT file is used in a 
<track>
 element where the attribute 
kind
 is chapters then you cannot use tags.
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>
).
Example 14 - Class tag
<c.classname>text</c>
Example 15 - Italics tag
<i>text</i>
Example 16 - Bold tag
<b>text</b>
Example 17 - Underline tag
<u>text</u>
Example 18 - Ruby tag
<ruby>WWW<rt>World Wide Web</rt>oui<rt>yes</rt></ruby>
Example 19 - Ruby text tag
<ruby>WWW<rt>World Wide Web</rt>oui<rt>yes</rt></ruby>
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:

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>
tag is for italics.
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:
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
interface
TextTrack
interface
Notes

Prior to Firefox 50, the 
AlignSetting
enum (representing possible values for VTTCue.align) incorrectly included the value "
middle
" instead of "
center
". This has been corrected.
WebVTT was implemented in Firefox 24 behind the preference 
media.webvtt.enabled
, which is disabled by default; you can enable it by setting this preference to 
true
. WebVTT is enabled by default starting in Firefox 31 and can be disabled by setting the preference to 
false
.
Prior to Firefox 58, the 
REGION
keyword was creating 
VTTRegion
 objects, but they were not being used. Firefox 58 now fully supports VTTRegion and its use; however, this feature is disabled by default behind the preference 
media.webvtt.regions.enabled
; set it to 
true
to enable region support in Firefox 58. Regions are enabled by default starting in Firefox 59 (see bugs bug 1338030 and bug 1415805).

Credits