css-variables are ignored

[ppseprus]

I believe, css-variables are ignored by doiuse.

Steps to reproduce

1. Create the test.css file with the following content

:root {
  --foo: red;
}
div {
  background-color: var(--foo);
}

2. Then, run the following command: doiuse --browsers "ie 6" test.css
   (for details: https://caniuse.com/#feat=css-variables)

Current behaviour
The following error is returned: test.css:1:1: CSS3 selectors not supported by: IE (6) (css-sel3)
https://caniuse.com/#feat=css-sel3

Expected behaviour
An error for the following feature is also returned by doiuse:
https://caniuse.com/#feat=css-variables

Note
I've also tried the following code, where no error was returned.

div {
  --foo: red;
  background-color: var(--foo);
}

[clshortfuse]

Fixed in v5.0