Hard to say. Could be rendering issue, or something to do with your CPU emulation (...which in turn would cause a rendering issue). Have you tried running other test ROMs for basic graphical output (or for that matter, commercial ROMs?) I would suggest trying Imran Nazar's Opus 5 demo (https://github.com/Two9A/jsGB/tree/master/tests
). It's what I cut my teeth on way back when I started GB/GBC emulation. Try to at least get that displaying the BG correctly, then move on to several title screens for commercial games. Super Mario Land, Tetris are good ones to start with, since they don't really do anything fancy before displaying things on-screen.
Also, for individual tests, it should be easy enough to run it through a debugger (look into BGB) and see if you're doing everything correctly. If not, then you definitely have a CPU issue, and you should look into what behavior the test considers accurate. Of course, you should have a debugger in your own emulator. If not, code one right away! I can't stress enough how important this is