player probably the first browser based mp3 player which could load local files

This used the new <input type="file" directory>
feature to load entire directories from the local computer. It uses FileReader
and JSID3v2 in order to read song metadata.
The interface is dominated by a gigantic search bar, and it has a fairly minimal interface for queuing songs.
related
MusicAlpha
reverse-engineered Google Music uploader

ExFM Extras
an extension to an extension to read ID3v2 metadata when importing music

JS ID3v2
a javascript library for reading ID3 tags from MP3 files

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