WebVTT

WebVTT (Web Video Text Tracks) is a World Wide Web Consortium (W3C) standard for displaying timed text in connection with the HTML5 <track> element.
The early drafts of its specification were written by the WHATWG in 2010 after discussions about what caption format should be supported by HTML5—the main options being the relatively mature, XML-based Timed Text Markup Language (TTML) or an entirely new but more lightweight standard based on the widely used SubRip format. The final decision was for the new standard, initially called WebSRT (Web Subtitle Resource Tracks). It shared the .srt file extension and was broadly based on the SubRip format, though not fully compatible with it.
The prospective format was later renamed WebVTT. In the January 13, 2011, version of the HTML5 Draft Report, the <track> element was introduced and the specification was updated to document WebVTT cue text rendering rules. The WebVTT specification is still in draft stage but the basic features are already supported by all major browsers.
Main differences from SubRip
WebVTT's first line starts with WEBVTT after the optional UTF-8 byte order mark
There is space for optional header data between the first line and the first cue
Timecode fractional values are separated by a full stop instead of a comma
Timecode hours are optional
The frame numbering/identification preceding the timecode is optional
Comments identified by the word NOTE can be added
Metadata information can be added in a JSON-style format
Chapter information can be optionally specified
Only supports extended characters as UTF-8
CSS in a separate file defined in the companion HTML document for C tags is used instead of the FONT element
Cue settings allow the customization of cue positioning on the video
Compatibility
Firefox implemented WebVTT in its nightly builds (Firefox 24), but initially it was not enabled by default. The feature had to be enabled in Firefox by going to the "about:config" page and setting the value of "media.webvtt.enabled" to true. YouTube began supporting WebVTT in April, 2013. As of July 24, 2014, Mozilla has enabled WebVTT on Firefox by default.
Subtitles in a .vtt file show online, but not when stored on a local drive.
Example of WebVTT format
A sample file from the W3C captioning Roger Bingham interviewing Neil deGrasse Tyson:
WEBVTT
00:11.000 --> 00:13.000
<v Roger Bingham>We are in New York City
00:13.000 --> 00:16.000
<v Roger Bingham>We're actually at the Lucern Hotel, just down the street
00:16.000 --> 00:18.000
<v Roger Bingham>from the American Museum of Natural History
00:18.000 --> 00:20.000
<v Roger Bingham>And with me is Neil deGrasse Tyson
00:20.000 --> 00:22.000
<v Roger Bingham>Astrophysicist, Director of the Hayden Planetarium
00:22.000 --> 00:24.000
<v Roger Bingham>at the AMNH.
00:24.000 --> 00:26.000
<v Roger Bingham>Thank you for walking down here.
00:27.000 --> 00:30.000
<v Roger Bingham>And I want to do a follow-up on the last conversation we did.
00:30.000 --> 00:31.500 align:right size:50%
<v Roger Bingham>When we e-mailed—
00:30.500 --> 00:32.500 align:left size:50%
<v Neil deGrasse Tyson>Didn't we talk about enough in that conversation?
00:32.000 --> 00:35.500 align:right size:50%
<v Roger Bingham>No! No no no no; 'cos 'cos obviously 'cos
00:32.500 --> 00:33.500 align:left size:50%
<v Neil deGrasse Tyson><i>Laughs</i>
00:35.500 --> 00:38.000
<v Roger Bingham>You know I'm so excited my glasses are falling off here.
Other features
In June 2013, an example was added to the specification that included a new "region" setting. This feature is supported since Firefox 59 and Safari 14.1 (14.5 on iOS) but not in any other browser.
References
External links
WebVTT Standard
Mozilla's developer page concerning WebVTT implementation
Subtitling
Subtitle file formats
World Wide Web Consortium standards