[ac1982]

Transcript text is written directly into WebVTT cue markup. Literal <b>tag</b> loses its tags in a browser, and literal & displays as &. This affects normal, word-by-word, and karaoke captions.

Escape transcript text at the WebVTT rendering boundary, before adding the tool's own karaoke <b> tags. Caption JSON retains raw text. SRT retains its existing rendering because FFmpeg's SubRip decoder does not decode HTML entities; applying WebVTT escaping there would display entity strings literally.

Validation:

• Three WebVTT style regressions fail before the fix and pass afterward; a JSON preservation control passes.
• Three real FFmpeg SRT-to-ASS tests verify ampersands remain literal in normal, word-by-word, and karaoke captions. These catch the SRT regression found during review.
• All 11 escaping/timestamp tests pass.
• Chromium loaded generated WebVTT tracks in all three styles. getCueAsHTML().textContent preserves the original literal text, and karaoke bolding still applies to the active word.
• Combined locally with fix(subtitle): preserve literal backslashes in correction values #718, fix(trimmer): isolate concat workspaces and preserve source files #719, and fix(audio-energy): handle fractional video duration windows #720: all 25 related tests pass.

The cue-text rules are described in https://www.w3.org/TR/webvtt1/#webvtt-cue-text.

Checked existing subtitle/markup/escaping issues and PRs. #538 handles CJK spacing; #718 fixes regex replacement interpretation in corrections. This change concerns WebVTT serialization and is independent of those fixes.

[calesthio]

Thank you for fixing literal text in WebVTT. Reviewed the complete diff against main 63d1087a: escaping occurs at the VTT boundary before karaoke markup, while raw JSON and SubRip behavior are preserved. This agrees with the WebVTT cue-text rules (https://www.w3.org/TR/webvtt1/#webvtt-cue-text).

Validation: isolated offline run of tests/tools/test_subtitle_escaping.py and tests/tools/test_subtitle_timestamps.py — 11 passed, including actual FFmpeg SRT decoding controls.