I'd prefer the rows, columns and diagonals to have their current sums visible next to the desired totals. Right now, it's tedious to have to add all the numbers up for each one.
A nice feature would be "locking" black squares, or otherwise marking them somehow, to distinguish them from squares you may still switch in the future.
The HTML structure is very problematic. You use a <table>, even though semantically the game grid is not a table of data. I would have used a <div> with display:grid , though this is not a big deal. The active cell is a <div> with onclick, which means the cursor doesn't show it as clickable and I can't even tab to it so keyboard play is impossible. There are ways to make a <div> look clickable, but it's easiest to just use <button> .
I liked the game as it is in terms of hints and assistance.<p>The only thing I at one point missed was the undo action. I accidentally fat-fingered two correctly placed squares, and although it was frustrating at first, in the end I appreciated the additional challenge.
It's a fun little game - the minimalistic UI/UX suits it well. I'd consider lowering the volume on the music. It doesn't really feel like it suits a casual puzzle game either.