Is it cssnext or CSSNext or CSSnext?

The official name is cssnext, which should never be capitalized, especially not at the start of a sentence, unless it is being displayed in a location that is customarily all-caps (such as the title of man pages).

Why ?

function nameify(string) {
  return string
    .replace(" ", "")

nameify("CSS Next")
nameify("CSS next")
nameify("css next")

// all the results above are just the same
// -> "cssnext" !