Hi Leen,
The Step parameter is an integer so it's normal that 0.5 doesn't work.
Just bumped in this same problem and worked around it by cloning the whole Rangeslider widget from SilkUI (also, added an extension https://simeydotme.github.io/jQuery-ui-Slider-Pips/#installation).
What I don't understand, is why step is set to be an integer parameter in first place. Underlying implementation is using decimal step value.
Bug/improvement idea considering platform?
I Agree with Mikko.
If the inputs MinValue, MaxValue and Step were Decimal the component would still work and accomplish its purpose for decimal scales as well as for integer scales... For now the easiest solution for developers is to clone the component and change the datatype of these attributes... which is kind of nonsense.