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.

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.