just adding some comments
the slider button was looking very different in Firefox and Chrome, fixing that...

      # Had to change these “0.8” and “0.9” coefficients
      # they were originally both 0.5, which caused
      # any "square" slider button to be a degenerate
      # line of zero length. Chrome and Safari optimise
      # that by not drawing it, which was making it
      # look different from Firefox.