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It is not too surprising that many HTML or JS based watermarking schemes are very easy to remove, some even via a script. For watermarking schemes to be effective, the watermark payload needs to be embedded in the content. Here's one approach that works for images: https://github.com/tim-janik/imagewmark/

Another that works for audio content is: https://github.com/swesterfeld/audiowmark

Incidentally, the latter also supports applying the watermark payload to HLS segments.