If you look at the source of a WordPress back-end page, you'll see it has a body class of
no-js. Immediately after the opening
body tag is a small script which replaces
js (you can see the amended class with Firebug / Inspector).
This plugin recreates the same effect, but for the front-end of Genesis Framework child themes.
Usually, yes, but it's a fairly small script, so does not block rendering of other content for any noticeable length of time.
Doing it immediately also reduces a flash of incorrectly styled content, as the page does not load with
no-js styles, then switch to
js once everything has finished loading.