One of the most user-friendly formats you can convert your TTML file into is VTT.
If you need to convert your files from TTML to VTT, consider using GoTranscript's online subtitle converter. This subtitle converter is free, and you do not need to register for an account to use the converter.
Additionally, you won't need to install any software since the converter runs on a web page. You can use this tool from any device with internet connectivity and a web browser. Here's how to use the GoTranscript subtitle converter:
Enjoy hassle-free TTML to VTT file conversion with the GoTranscript subtitle converter. The subtitle format conversion tool is fast and absolutely free, and you can use it at your convenience to convert as many files as you want.