CSSLint
Unknown property background-position-x/background-position-y
It seems parser-lib is unaware of background-position-x and background-position-y which results in a warning:
Unknown property 'background-position-x'. Properties should be known (listed in CSS3 specification) or be a vendor-prefixed property. (known-properties)
Could you fix it?
background-position-x and background-position-y are actually nonstandard, although they apparently have been newly approved for CSS v4. See: http://stackoverflow.com/questions/9653685/is-background-position-x-background-position-y-a-standard-w3c-css-property
It's "on standards track", we'll say, but the official spec will be https://drafts.csswg.org/css-backgrounds-4/#background-position-longhands and it currently has big warnings plastered across it: "This spec is not yet ready for implementation. It exists in this repository to record the ideas and promote discussion." and "ISSUE 4: This section is still being worked out. The tricky thing is making all the start/end keywords work sanely." It seems premature to implement this at the current time, and so I've applied the "not in latest official spec" label to this issue for now.