Scratch

Why don't custom blocks work online?

Lots of people would like Java/Flash players to support custom blocks, such as Mod Share, Kitcat, and other project sharing websites. The reason is that both players don't read Squeak/Smalltalk. They are just programmed to read the project's scripts, sprites and sounds, and run it. When a new block on Scratch is made, the Scratch Team would have to add the blockspec and code to both players, and then run a couple of tests.

Why don't custom blocks show when downloaded?

The reason for that is that custom blocks would have to be programmed in the other user's computer to. Because when Scratch finds a block that's not supported, it automatically makes the block a red command block with the text "Obselete". Then people started making mods (short for Scratch modification), to share new blocks with others.