Add Subtitles to a WebM Video

Who makes WebM files

WebM is the open video format Google introduced for the web, built on the Matroska container. You usually have one because something in a browser recorded it: a screen-capture extension, an online webcam or voice recorder, an interview tool, or a website's own record button. Chrome and Firefox write WebM by default when a web page records video, so anything built on that feature produces it. OBS Studio and some desktop recorders can save WebM too, and the format turns up in downloads from sites that serve it.

Inside, the picture is VP8, VP9 or AV1 and the sound is Opus or, in older files, Vorbis. Those codecs are royalty-free, which is why browsers took them up, and also part of why some Apple apps, editing programs and upload forms are less welcoming to them.

Codec by codec

VP8

The original WebM video codec, still used by some browser recorders. Chrome, Edge and Firefox decode it.

VP9

The usual choice for screen and webcam recordings now. Chrome, Edge and Firefox decode it; Safari's support came later and is less consistent.

AV1

Newer, more efficient and more demanding to decode. Current Chrome, Edge and Firefox handle it; on older machines and in Safari it depends on the hardware.

Opus

The standard WebM sound. MP4 can store it, so the editor copies it into the download as it is.

Vorbis

The older WebM sound, also copied unchanged.

If the browser can't decode the picture, the editor says so as soon as the file has been read, names the codec and leaves Render video unavailable. Open the same file in Chrome or Edge, or convert it first.

Subtitling a WebM recording

1. Open the editor in Chrome, Edge or Firefox and choose the .webm file.
2. With "Write the subtitles automatically with AI" switched on, the sound is extracted and sent for transcription as soon as the file loads. Switch it off first if you have an SRT or VTT to import or would rather type.
3. Wait until the bar above the preview shows the frame size, which confirms the recording can be rendered.
4. Work through the lines: correct product names and jargon, fix timing on the timeline, and use Split where a sentence runs on too long.
5. In Style, choose a preset and position. For a screen capture, make sure the text won't sit over the part of the interface you're demonstrating.
6. Click Render video, check that the summary says the sound will be copied, render, and download the MP4.

WebM in, MP4 out

The editor always writes MP4, so a WebM changes container on the way through. The picture is re-encoded, normally to H.264, which is why the subtitled copy plays on iPhones, in most editing apps and on upload forms that refused the WebM.

The sound is treated differently. MP4 can carry Opus and Vorbis, so the editor copies the recording's sound into the MP4 untouched rather than converting it, and the summary reads "Opus — copied without re-encoding". The audio stays exactly as recorded, and Chrome, Edge, Firefox and VLC play it without fuss. Some players are pickier about Opus inside an MP4, older Apple software and smart TVs among them. If the download plays silently somewhere, convert only the sound and leave the picture alone: ffmpeg -i demo-subtitled.mp4 -c:v copy -c:a aac demo-aac.mp4. The Opus codec explains where it's supported.

Otherwise the usual render rules apply: the same resolution and frame rate as the recording, a generous bitrate and no watermark.

Recordings that came straight from a browser

A browser writes a recording on the fly, and many of those WebM files lack the index that tells a player how long the video is and where each moment sits. Players then show no duration, or seek slowly. The editor works out the length by reading the file itself, so the timeline is correct, but scrubbing the preview of a long recording can still feel sluggish. Rewriting the file with FFmpeg adds the missing index without touching a pixel: ffmpeg -i recording.webm -c copy fixed.webm.

Two other habits of browser recordings are worth knowing. The frame rate often varies with what's happening on screen, and the voice may be quiet if the microphone level was low. Neither blocks a render, but faint speech makes AI subtitles less reliable; screen recording audio settings helps next time you record.

Keeping a WebM for your own site

If the video lives on your own web page, you may not need to burn anything. The HTML video element plays WebM and reads WebVTT captions through its track element, so you can keep the original recording, export a VTT from the editor and let viewers turn captions on and off. Subtitles for HTML5 video shows the markup. Burn them in when the clip is going somewhere that ignores caption files: a chat app, a social feed, a slide deck.

For placing captions over interfaces and code, subtitling screen recordings has specific advice, and when the important part of the screen is its bottom edge, moving subtitles to the top shows the settings. If all you need from the recording is its text, WebM to text produces a transcript instead.

Three WebM recordings

About the subtitle editor on this page

Opens

MP4, MOV, M4V, WebM, MKV videos your browser can decode

Subtitles from

AI (from the speech, optionally translated into one of 21 languages), an SRT or VTT file, or typing them in

You download

An MP4 with the subtitles drawn into the picture at the original resolution and frame rate, plus SRT and VTT files

Cost

Editing, styling, rendering and SRT/VTT export are free, with no account and no watermark. AI subtitles cost 1 credit per minute (5-credit minimum per video); signing in with Google gives 100 free credits and accounts get 10 free credits a day

Privacy

The video never leaves your device and is rendered in your browser. Only for AI subtitles is the audio sent, over HTTPS, and it is deleted within 30 minutes of delivery

Rendering needs

Chrome or Edge 94+, Safari 16.4+, or Firefox 130+ on a computer. Editing and SRT/VTT export work in any current browser