Keen Eye

Pay close attention… http://challenge.rgbsec.xyz:8376/

Solution

In the source code, the first thing to note is that all errors are suppressed by

window.onerror = function (message, url, lineNumber) {
	return true;
};

This leads to the inference that there is some error in the page that is being ignored. The comment clearly says that the form does nothing so by downloading the source code locally and changing the error function to alert instead, it is seen that there is some error in the CDN: https://cdn.jsdelivr.net/npm/poppers.js@1.16.1/dist/umd/popper.min.js

On visiting this CDN and searching for rgbctf, the flag is obtained.

Flag

rgbctf{n073_7h3_d1ff}