Does Not Correctly Parse a '--' Ident

[Alexhuszagh]

Using the web service, with the following example, we get a CSS parse error:

.myButton {
  --: 0px;
}

The following warning is produced:

Parse Error --: 0px; 

However, according to the current CSS Level 3 syntax, -- is a valid identifier start, and all identifier starts do not need to be followed by other tokens in order to form a valid <ident-token>. I understand this is a fairly unusual edge-case.

Other standard conforming examples below all parse:

.myButton1 {
  --background: 0px;
}

.myButton2 {
  -background: 0px;
}

.myButton3 {
  -\21: 0px;
}

.myButton4 {
  -\ : 0px;
}

.myButton5 {
  -\:: 0px;
}

.myButton6 {
    -\AF12 a123: 0px
}

While invalid CSS fails:

.myButton1 {
  -\
: 0px;
}

In short, the CSS grammar for parsing <ident-tokens> is correct in all other examples, and fully conforms to the specifications, including escape tokens. However, -- is not properly handled.

[ylafon]

fixed (partially) by #316