CSS user-select goes CSS_INVALID_PROPERTY_NAME_ERROR

[humphd]

Moved from https://github.com/mozilla/thimble.webmaker.org/issues/481:

@pamelafox filed:

Attempting to use this non-standard property results in the invalid property name error:
https://developer.mozilla.org/en-US/docs/Web/CSS/user-select

I can see where to add it in the code (cssProperties), but not sure if that'd conflict with philosophy about which properties "exist". Thoughts?

and @Pomax wrote:

this would be an issue with https://github.com/mozilla/slowparse, adding "user-select" to the list of known CSS properties should be all that's required (although adding a test case so that it's provably accepted would be good too of course!)