add declarations validation

[egoholic]

• declaration property is known/standard
  For example, color, background-color,width, etc. are valid, some-prop, --custom-prop-x, -moz-appearance and -webkit-appearance* are not valid.
  Custom browser properties/declarations are not valid yet. They will be added in a separate task.

• declaration value has proper type
  For example, color: white, color: #10ffc5, are valid, color: rgba(0,64,128,0.314) - is not valid.
  CSS functions will be added in a separate task.