Floor is Lava Challenge

This simple mod adds a "the floor is lava"-style challenge to the game:

When you spawn, the block you are standing on is recorded. If you ever step onto a different type of block, you are instantly killed.

For example, if you spawn on grass, walking onto dirt or stone will kill you.

Cheese Trick

Note that you can cheese the mechanic a little bit by leaping over to a new block type and quitting the game in midair, since the "spawn block" is not saved.

The mod is public domain and extremely simple. Feel free to make your own variant! The project sources are released in a ZIP file alongside the mod release.