It seems that as of some time between July 10, 2016, and July 31, 2016, the "multiple" attribute was removed from the input[type=range] spec, and is now listed as "does not apply".
Here is the latest web archive page with that text:
http://web.archive.org/web/20160710230742/https://html.spec.whatwg.org/multipage/forms.html#range-state-(type=range):attr-input-multiple-3