Minecraft wiki entry for Classic_0.0.23a_01_(remake), associated text: When exiting and entering the world, the top textures of all blocks that were at altitude became invisible.
The human mob model is glitched.
Unloaded chunks turned white.
The sea level in the game world did not match the sea level outside the world.
FPS counter is inaccurate on some browsers and will sometimes display bogus/random numbers.
Zooming in and out will sometimes cause a huge drop on FPS, this will cause an performance impact, and this will make all blocks disappear, and display an error icon.