Tech stack

Every technology the kit uses, what it does here, and why it was chosen. Versions are the ones the kit was built and tested with in October 2026, on Windows 11.

At a glance

The kit's own code is Apache-2.0. It bundles none of the above: each is installed on your machine or downloaded into the cache. See NOTICE.

Runtime: Bun

What it does. Runs the TypeScript in src/ directly, with no compile step. Installs dependencies (bun install, bun.lock). Provides fetch for the two downloads the kit makes. The tests run on Vitest, started with bun run test.

Why. A video file is TypeScript, and with Bun the user's video.ts just runs. The package exports its source to Bun, so a clone needs no build.

Node. The kit also runs under Node 22.18 or later, from dist/, which bun run build compiles with the TypeScript compiler (no bundler). The launcher bin/showcase.js picks src/ under Bun and dist/ under Node. This is what an npm install uses. Node reads a project's own video.ts by stripping its types.

What to know.

• The kit uses only Node-compatible APIs (node:fs, node:child_process, node:http), not Bun-only ones, so it is not locked to Bun.
• Playwright under Bun on Windows had launch failures in older Bun versions. On 1.4.2 it launched and captured correctly in 6 of 7 trial runs. The one failure was a cold first start that delivered no frames; the kit reports that with a "render again" message instead of writing an empty video.
• The kit installs from npm. To use a local clone from another project, while changing the kit, use bun link: installing the folder with file: failed with a permissions error on Windows.

Language: TypeScript

Strict mode, with noUncheckedIndexedAccess. defineVideo exists only to give a video file type checking and editor completion: a wrong option name or a missing say is an error in the editor, not a failure four minutes into a render. tsc --noEmit is the type-check; nothing is emitted.

Browser automation: Playwright

What it does.

Why. It is the one tool that drives the app, captures the frames and draws on top of them. The screencast API with per-frame callbacks arrived in Playwright 1.59; that is the minimum version.

What was not used, and why.

Voice: Kokoro

What it does. audio/tts.ts sends the lines that are not yet cached to python/kokoro_tts.py as JSON on standard input. The script loads the model once and writes a 24 kHz mono WAV per line.

Why. It is free, runs on a laptop CPU in about three seconds per line, needs no account and uploads nothing. A render never fails because of someone else's quota. The voice quality is good enough that the first reviewers commented on the sound effects and not on the voice.

Why Python, in a TypeScript project. kokoro-onnx is a Python package. Rather than reimplement its text-to-phoneme pipeline, the kit keeps a 25-line script and runs it in its own virtual environment (~/.cache/kwartz-showcase/kokoro-venv), created by showcase setup. Python is needed for the voice and nothing else; draft renders work without it.

Model files. kokoro-v1.0.onnx (about 325 MB) and voices-v1.0.bin (about 28 MB), from the kokoro-onnx GitHub release. Downloaded once.

Media: FFmpeg

One program does all of these jobs.

Why frames as a list with durations. The browser paints when something changes, not 30 times a second. Giving FFmpeg each frame and how long it lasts keeps real time exactly, and a still screen costs one frame instead of thirty a second.

Output. 1920x1080, 30 fps, H.264 (yuv420p, plays everywhere), AAC 48 kHz, about 1 Mbit/s for a typical screen video (a three-minute video is about 28 MB).

Motion: HTML and CSS

Every overlay and card is a string of HTML with inline styles and CSS keyframes. There is no animation library in the kit.

Why. An agent writes HTML and CSS well, the result needs no bundling, and it is captured by the same screencast as the app. Two easing curves cover everything: cubic-bezier(.2,.8,.2,1) for arrivals and cubic-bezier(.45,0,.2,1) for moves.

For a card with a complex timeline, a video file can load an animation library such as GSAP into its own HTML. The kit does not need it and does not depend on it.

What the kit depends on

dependencies
  playwright 1.64.0        the only runtime dependency

devDependencies
  typescript ^5.6          type-check
  @types/bun               types for Bun's APIs
  vitest ^5.0              the test runner
  @vitest/coverage-v8      coverage reports

One runtime dependency is on purpose. FFmpeg, Python and the voice are programs on the machine, checked by showcase doctor, not packages.

Considered and not chosen

System requirements