Plays inside your own copy of Hytale, on your device, not in a browser tab. Opens curseforge.com in a new tab.
How to play
Have your own copy of Hytale installed: the mashup plays inside it.
Download it from its release page on CurseForge.
Install it as its page says. What it asks for is just below, in the list's words.
Start the game the way its page says: some mashups come with a launcher of their own.
What it asks for
Hytale (game version 0.6 for the current file). Downloads the shareware DOOM1.WAD at startup (needs internet); Freedoom is bundled in the JAR (/doom new --mode freedoom).