Look, this is a web-based calculator. You can't use something like this without JavaScript enabled, or else I'd have to write a server that would use more computation to encrypt the traffic than to actually perform the calculations necessary to figure out how long an antenna should be.
You can audit the source code for this application here: GitLab
Enter your desired frequency in the Frequency field, and select a unit (Default is MHz) from the drop down, then click "Calculate". It's that simple!