If someone wants to try music, start with playing an instrument and then pick the theory. If in physics theory says how the world works, music theory is mostly about labeling things that sound good vs noise. And it's hard to get the words without playing first.
Focused more on practitioners. Includes exercises with MIDI/USB piano keyboards in the browser: https://www.teoria.com/en/exercises/
These resources also can be useful:
1. https://www.musictheory.net/lessons (exercises included)
2. http://www.earbeater.com/online-ear-training (ear training exercises)
3. https://www.iwasdoingallright.com/tools/ear_training/online/ (ear training)
4. https://trainer.thetamusic.com/
5. http://music.stackexchange.com/
N. Wikipedia is a good reference. E.g., https://en.wikipedia.org/wiki/Glossary_of_musical_terminolog...
If someone wants to try music, start with playing an instrument and then pick the theory. If in physics theory says how the world works, music theory is mostly about labeling things that sound good vs noise. And it's hard to get the words without playing first.