THIS IS THE MOST POWERFUL FIX EVER (FOR A WEBSITE).
I debugged my site for nearly an hour for some css loading problem. Located the seemingly problem finally but was unable to fix it due to code obfuscation. The project is open-source but I didn't bother to go through the code, which was a lazy and unresponsible move, but was also demonstrated a wise one later...
Anyway at last I decided to contact the developer.
And if you understand the title, you know of course there is no problem with the code itself.
It's just CACHE! One Ctrl + F5 fixed everything.
Credit to 淮城一只猫.