I developed a C# C64 emulator some years ago (in 2015, I think), but C# is so 2010.
So I took my beloved javascript and, re-using the 6502 core from almostNES, a C64 emulator was ready in a few days. Then I spent some more days trying to improve the compatibility.
This new emulator has SID sound, sprite collisions, sprite priorities, digis, and more (things that the C# version had not). SID was so ahead for its time (remember that it was made in 1982, so it's a chip that is more than 40 years old).
The javascript code should be self-explanatory. What really helped while writing the source code were a couple of books that describe the 6502 and the graphic/sound chips of the C64 ("Programming the 6502" by Rodnay Zaks and "Commodore 64, la grafica e il suono" by Rita Bonelli and others).
Some screenshots:
The online, up-to-date version is here: http://dantonag.it/almostC64/index.html