Progressive levels. Find the flag for the current level and submit it. Use View Source / DevTools / Console.
Current Level: 1 / ∞ (extendable)
The password is hidden in the HTML source.
Something is hidden on the page.
Open the Console and look around.
window or look for variablesDecode this: RkxBR3tsZXZlbDRfYmFzZTY0X2lzX2Vhc3l9
Look at the JavaScript that runs on this page.
Check storage (Application tab → Cookies / Local Storage).