I used this to implement <yes-script>, the opposite of <noscript>, to be able to designate sections of a page to be hidden if JS was disabled. You can of course do the same thing with classes, but custom tags are fun.

https://github.com/aaviator42/yes-script

By the way, you can also do this pure CSS using the scripting media feature: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/A...