Minecraft wiki entry for Classic_0.0.23a_01_(remake), associated text: General
Since this is a remake and not a port, the code, apart from the terrain generation code which is the only ported code in the game, is completely different from the original and as such there may be many minor gameplay inaccuracies.
Grass blocks seem to turn into dirt when in shade.
The remake version has an autosave feature.
Because of this, reloading or exiting then reopening the website on the same browser will load the same world and seed, along with player-made modifications.
Physics are very different from the original.
New level format.
Block IDs in this remake do not line up with the original.
Human mobs
They have a strange appearance compared to the original version's humans, likely unintentionally as the result of an update.
They have a different, less random and frantic AI.
They can cross the world border, revealing that unlike the original, the bedrock and water beyond the border are actually tangible and are actual blocks rather than a rendered flat plane.
They will fall through the world after going too far, however.
They can be spawned in multiplayer, although they will only appear on the client side.
GUI
The blue gradient, seen in the inventory, pause menu and other menus in the original, is replaced with simply a transparent grey colour, reminiscent of modern Minecraft.
The red gradient, seen on the crash/kicked from server screens in the original, is replaced with the same dirt background as seen on the generating level screen in both the original and the remake.
The font is slightly different, and is also rendered anti-aliased on most browsers.
The latter is because the font uses a .ttf fonts file rather than a .png image like the original.