This website is dual-licensed:

I want to use this website, what should I do?

(IANAL) Here is the website's source code. Please read tldrLegal's summary of the CC BY-NC-SA 4.0 and the GPLv3 and understand what you can, cannot, and must do when working with these licenses.

It is ultimately up to you if you choose to act upon this; at the very least please consider adding a simple attribution with a link back to this website, e.g. "Based on content from".