The keyboard instructions are not in the game, but rather on the Itch page just below the game window.
Here's the entirety of the text related to keyboard controls:
Keyboard Controls
I am investigating an issue with keyboard controls. At this time, due to browser security constraints, you must use the tab key to focus-cycle the game in order to enable keyboard controls.
I recommend just holding tab for a second to get through everything on the page. Once enabled, you may use the arrow keys for movement and the S/D/F keys to block, light attack, and heavy attack.