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

• single leading dash is used for vendor prefixes
• double leading dash is used for defining custom properties.

2 Defining Custom Properties: the '--*' family of properties

A custom property is any property whose name starts with two dashes (U+002D HYPHEN-MINUS), like --foo. The production corresponds to this: it’s defined as any valid identifier that starts with two dashes.

An example from W3C:

:root {
  --main-color: #06c;
  --accent-color: #006;
}
/* The rest of the CSS file */
#foo h1 {
  color: var(--main-color);
}

It's worth noting that CSS variables are implemented in Firefox 31 and newer.