Fix CSS syntax validator

Currently [CSS syntax validator](https://csstree.github.io/docs/validator.html) is using its own rules to validate CSS. These rules are outdated and confuses users (https://github.com/csstree/csstree/issues/171). So the rules should be fixed or replaced with [csstree's validator](https://github.com/csstree/validator).

An alternative option to reject the validator's page and redirect to [CSS syntax reference](https://csstree.github.io/docs/syntax.html) 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.