Fix CSS syntax validator

[lahmatiy]

Currently CSS syntax validator is using its own rules to validate CSS. These rules are outdated and confuses users (csstree/csstree#171). So the rules should be fixed or replaced with csstree's validator.

An alternative option to reject the validator's page and redirect to CSS syntax reference where will be a functionality to validate CSS syntax. This option looks more promising to me, because in addition to CSS validating according to existing syntax definitions, I would also like to add the ability to supplement or redefine syntax definitions.

[RobertAKARobin]

I'm one of the developers who got confused! CSS variable declarations appear to make the validator just stop working — when present (e.g. --color: green;) the validator doesn't process input.

[lahmatiy]

@RobertAKARobin The issue you are describing has just been fixed.