fix merging rules, css selector validation, experimental prefix remov…

[tbela99]

• fix merging rules
  merging

.a {
    display: node;
}

.b {
    display: node;
}

.c {
    display: node;
}

would produce

.b,.c {
    display: node;
}

• experimental CSS prefix removal
  • declarations
  • declaration values
  • exclude -webkit-* gradients
• css selector validation
  • pseudo element
  • partial pseudo class validation. does not validate parameters
  • attribute selector
  • combinator
  • simple selector
  • nested selector
  • strict mode: allow unknown items such as pseudo classes
    • allow unknown pseudo classes
    • allow unknown attribute selectors
• strip universal selector when possible