CSS is much tighter linked to HTML than a timed text format. If your UA happens to not support TTML, only one feature will be missing, i.e. timed text on your video. That doesn't destroy your Web page. But lack of CSS support does.
CSS is by design, optional. I would hope a captioning format is not, since captions are somewhat essential content information.
-- Anne van Kesteren http://annevankesteren.nl/
