WebVTT

WebVTT (Web Video Text Tracks) is a World Wide Web Consortium (W3C) standard for displaying timed text in connection with the HTML5 codice_1 element.
The early drafts of its specification were written by 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 codice_2 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], thecodice_1 tag 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.
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.
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 but not in any other browser.