Snapshot 21w13a (released in the 13th week of 2021, hence the name) is the latest development version for the 1.17 Caves & Cliffs update.
This snapshot adds the quirky goat mob to Java Edition, after it was added to Bedrock last year.
Goats spawn on top of mountains, and can be milked with a bucket and bred using wheat. Holding wheat will make them follow you.
Unlike their Bedrock equivalent, goats will not attack you. This will most likely change before the full release of 1.17.
Another new feature is the light block, which allows you to specify exactly what light level a certain space in the world will emit. There are fifteen different light levels available, which can be cycled through by right-clicking the block.
Light blocks are only available using a command as they are not a Survival gameplay feature.
Additionally, glow squids and axolotls will now spawn underground, inside flooded aquifer caves.
Other minor changes include horse armor protecting horses when inside powdered snow; the textures of axolotls have been tweaked; and large caves are slightly more common.
This snapshot also fixes over a hundred bugs, many of which regard rendering issues. Unused textures have also been purged.
For a visual overview of the major features in this snapshot, see the video below: