The TTML Profile for Internet Media Subtitles and Captions (IMSC) is the most promising candidate for a long awaited harmonized subtitle format for the Web. The open source library imscJS is not only a polyfill to render IMSC subtitles in the browser: as an open reference implementation it is a blueprint how to integrate IMSC subtitles in other open source projects e.g. in video players. The presentation will show how imscJS parses and renders IMSC subtitle files, where imscJS is already used and how the open source ecosystem for subtitles could benefit from it in the future.