graeme_p - 12:32 pm on Aug 7, 2013 (gmt 0)
Lucy, no, because you can detect visitors who did not have JS on at step 1.
Of course you need to look at new, not returning visitors.
The numbers for JS off are very low even on sites that work perfectly without JS.
It is also perfectly easy to notify visitors why the site is not working, and those who turn off JS should know how to turn it back on.