Review of TTML2 2nd Edition noted many potential fingerprinting vectors: #1189
(Whether those issues present a privacy risk depends on a clearer understanding of what information is revealed by content processors to whom.)

Addition of external font loading and the CSS font-matching algorithm could introduce those fingerprinting issues to IMSC 1.2.

Mitigations for fingerprinting in CSS are under discussion now in CSSWG and PING.

More info in email: https://lists.w3.org/Archives/Public/public-privacy/2020JanMar/0055.html