I'm wondering if the lack of JS on the Thormixer site is to speed up the interface and user experience or if it's also for user data security reasons. By the way, some mixers that are also represented on this forum, on the contrary, use JS on their sites. One of them even recommends enabling JS, if it is disabled in the user's browser.
The lack of JS is for speed as it allows the browser to load the website faster. No logs are to provide anonymity to the user who is using the mixer to get his transaction anonymous which makes perfect sense. Javascript is used to provide interactive features to the website and nothing more. Sometime excessive use can make a website more complex for a newbie to understand the interface.
Being some one who have some time ago learnt wen development, I want to confirm that what you said you absolutely correct.
Javascript (like you said) is used to make site interactive, while CSS is used to design a site, input, assign colors, special characters where they are needed, Javascript is just like the brain of the site, the brain is programmed to bahave in specific ways as a response to specific action from the user, Javascript can make a site pretty interesting to interact with, but it's always at the expense of speed, excessive use of it can not only slow down the site's speed, or cause newbies not to understand the site's interface, excessive use can also slow down the perfomance of the device being used to access such site, causing excessive and annoying lagging and hanging, most especially on devices with low memory.