Do custom CSS properties use one leading dash or two?

Question

#elem {
  -myCustom: 99;
}

OR

#elem {
  --myCustom: 99;
}

I have seen both of the above used in examples online.

Answer 1

Custom properties use one dash, by convention followed by the renderer/software.

For example:

-webkit-box-shadow

-moz-box-shadow ...

But it seems that there is a new feature implementing two dashes, this might be interesting for you:

http://www.broken-links.com/2014/08/28/css-variables-updating-custom-properties-javascript/