The 8-not-10 isn't a problem at all, just a little surprising and maybe a hint that the creator wasn't deeply experienced with sequencers.

To be honest that's fine. I sincerely think this is a pretty elegant implementation, maybe because you're looking at it with fresh eyes.

Traditionally the hardware versions of these things have a pretty clunky and awkward interface that seems designed to minimize the number of switches and dials rather than to be readily understandable. A lot of the "toy" software sequencers you'll find online are clearly modeled on the hardware sequencers - sometimes specific ones - and copy that clunky interface. The "here's a grid, toggle a button to play a sound on that beat" interface is much more intuitive and fun to play with.

I think this is really well done. The only idea for improvement that jumps out at to me would be the ability to drag the cursor with the mouse down to toggle multiple buttons at once rather than independently clicking on each one.

If you want to learn more about Music Theory, I highly recommend Adam Nealy's videos on YouTube. He takes a refreshingly relaxed, entertaining, and overall very enjoyable approach to music theory, in stark comparison to a lot of the extant music theory material available. Which is kind of ironic since one of the reasons people play music is to relax, entertain, and enjoy themselves: https://www.youtube.com/user/havic5

I also like Rick Beato, as he tends to do more song deconstruction, though sometimes he can come off as a bitter, old man (though most of the time it's endearing): https://www.youtube.com/user/pegzch

Finally, I also like Signals Music Studio. He takes a little more of a traditional approach, but still in the camp of positivity rather than criticism of "crappy modern music": https://www.youtube.com/channel/UCRDDHLvQb8HjE2r7_ZuNtWA

I love WebAudio stuff. It's so much fun to bang out a jam with some code. I could never get the gumption to get SuperCollider or any of the other systems working, because I was much more interested in the interactivity and sharing that are fundamental to the web.

Here's a multi-touch capable keyboard: https://codepen.io/SeanMcBeth/pen/meaQvj

Here's a game I made (about the consequences of following orders) that used nothing but WebAudio synthesis for the sound: https://www.seanmcbeth.com/yabs.html

Actually, I've been hacking on audio stuff in the browser before there was even a WebAudio API. Here's a synth I made that generated raw WAV files on the fly and played them in a standard <audio> tag (note: it pretty much only works correctly in Chrome): https://www.seanmcbeth.com/html5/synth/

Always keep playing.