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SRT is the simplest of the three subtitle formats, WebVTT is the W3C-defined format designed for text tracks on the web, and ASS is the choice when styling, positioning, or effects must be preserved. None of them is accepted everywhere. The right format is the one your destination player, platform, or editor will accept and render the way you intend, so check that before you export.
What each format is
SRT (SubRip Subtitle, .srt)
SRT is a plain-text subtitle format. Each cue is a numbered block containing a time range and the subtitle text, and blocks are separated by a blank line. The Library of Congress describes SRT as a format whose history and specification record is limited, and notes that it has limited information on SubRip’s development. It is nonetheless the format on which WebVTT was based, according to the Library of Congress description of the SRT format and the W3C WebVTT specification.
A typical SRT cue looks like this:
1
00:00:01,000 --> 00:00:03,500
The meeting starts now.
2
00:00:04,000 --> 00:00:06,200
Please mute your microphones.Note the comma before the milliseconds. That detail matters, because it is one of the most common differences that breaks a hand-edited file.
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WebVTT is defined by the W3C. The specification opens with the sentence: “This specification defines WebVTT, the Web Video Text Tracks format.” It is intended for external text tracks associated with HTML’s <track> element, and its stated uses include captions, subtitles, text descriptions, chapters for content navigation, and other metadata that is time-aligned with audio or video. The W3C document also says WebVTT files provide “captions or subtitles for video content, and also text video descriptions, chapters for content navigation, and more generally any form of metadata that is time-aligned with audio or video content.”
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A WebVTT file must begin with the line WEBVTT, and its timestamps use a period before the milliseconds:
WEBVTT
00:00:01.000 --> 00:00:03.500
The meeting starts now.
00:00:04.000 --> 00:00:06.200 line:85% align:center
Please mute your microphones.The second cue shows a cue setting, which controls where the text sits on screen. Cue settings, regions, and styling rules are part of what separates WebVTT from SRT. How fully a given player implements them is a separate question covered below.
ASS (Advanced SubStation Alpha, .ass)
ASS stands for Advanced SubStation Alpha. Its technical reference, maintained by the Subtitle Edit project, describes styles, layers, alpha-channel support, rotation, and effects. That reference is community-maintained documentation rather than an official standard, so treat its details as the project’s description of the format rather than a published specification.
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Unlike SRT and WebVTT, an ASS file defines reusable styles and then applies them to dialogue lines:
[Script Info]
ScriptType: v4.00+
[V4+ Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, Alignment
Style: Default,Arial,40,&H00FFFFFF,2
[Events]
Format: Layer, Start, End, Style, Text
Dialogue: 0,0:00:01.00,0:00:03.50,Default,,The meeting starts now.The same reference notes that different renderers may handle some override tags differently. This is the main practical risk of ASS, and it is discussed in the section on compatibility.
Side-by-side comparison
| Format | Best starting point | Main strength | Main trade-off |
|---|---|---|---|
| SRT (.srt) | Simple subtitle exchange when the destination accepts SRT | Simple, widely understood text cues; the basis for WebVTT | Less presentation structure than ASS or WebVTT; the Library of Congress record notes limited specification and development information (source) |
| WebVTT (.vtt) | Web text tracks and HTML <track> workflows |
W3C specification with cues, regions, cue settings, and uses beyond subtitles (source) | Rendering and supported features depend on the implementation; the destination must be verified |
| ASS (.ass) | Styled, positioned, or effects-heavy subtitles | Styles, layers, alpha-channel support, rotation, and effects (source) | More complex to write; renderers may handle some override tags differently |
The Subtitle Edit project also maintains a list of supported subtitle formats, which is useful when you need to know whether a given editor can read or write a format at all: Supported Subtitle Formats.
Which format to choose
Choose by the destination and by what the file has to preserve, in roughly this order:
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- The destination names a format. If a platform, client, or player specifies SRT, WebVTT, or ASS, deliver that format. Format preferences set by the receiver override everything else in this list.
- You are building an HTML5 video page with text tracks. Start with WebVTT and follow the W3C format requirements.
- The subtitles must keep specific styling, positioning, or effects. Use ASS, but only if the full playback chain has a renderer that handles the features you used.
- You only need plain, time-coded text and nothing more. SRT is the simplest file to write, read, and share by hand.
There is no universal winner. WebVTT is the natural fit for web text-track workflows, SRT suits simple exchange where it is accepted, and ASS suits styled subtitles when the playback chain supports them. Other formats may also be requested by a specific service, so the destination always decides.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Compatibility and what it does not prove
A format’s features do not prove that a particular TV, media player, platform, or upload service supports them. The W3C specification defines WebVTT, but whether a given player renders regions or cue settings is up to that player. The same applies to ASS: the reference notes renderer differences, so the same file can look different in two players. No universal compatibility matrix exists in the sources reviewed for this article, so avoid assuming that any one format works everywhere.
Converting between formats
Conversion is often necessary, and it can lose information. Converting an ASS file with positioning or effects to SRT, for example, will usually discard those features, because SRT has no equivalent structure for them. Converting SRT to WebVTT is generally the lower-risk direction because the two formats share a basis, but the result still needs checking. This is a practical inference from the documented differences between the formats; the article does not claim that any specific converter has been tested for quality.
How to check a file before delivery
- Open the destination’s current subtitle documentation and confirm the accepted formats and any file-size or encoding requirements.
- Export or convert the file, then open it in a plain-text editor. Confirm the SRT comma or WebVTT period in timestamps, the
WEBVTTheader for WebVTT, and the section headers for ASS. - Check that cue numbers (SRT) and timing run in order and that no cue ends before it starts.
- Play the file in the target player or platform preview, not only in the editor you used to create it. Look at line breaks, text position, and any styling.
- If the appearance differs, simplify the styling or switch to a format the destination renders more consistently, then repeat the playback check.
Which format is the W3C specification?
The W3C document referenced in this article is a Candidate Recommendation Draft published May 20, 2026, not a final Recommendation. Its content describes the format as the W3C currently proposes it, so check the W3C’s current status page before treating any detail as final.
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See also: the ASS format reference and the supported formats list, both maintained by the Subtitle Edit project.
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