feat(css): Add CSS and WebKit scrollbar specifications

[ExE-Boss]

Both @erikadoyle and @jpmedley have confirmed the Microsoft Edge and Google Chrome statuses respectively in mdn/browser-compat-data#2424 (comment) and mdn/browser-compat-data#2424 (comment).

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review?(@alrra)

[msftclas]

All CLA requirements met.

[alrra]

Cc: @FremyCompany @gregwhitworth

[FremyCompany]

The spec linked by @ExE-Boss and the description he gives it are not correct. The latest version of the spec has scrollbar-color: auto | light | dark and scrollbar-width: auto | thin | none and it is currently too early to signify either support or no support for the feature as the spec hasn't finished maturing yet. I recommend not merging this PR.

[FremyCompany]

The second part of the PR is however accurate, we could merge that:

  { 
    "name": "WebKit Scrollbars", 
    "category": "CSS", 
    "summary": "The ::-webkit-scrollbar CSS pseudo-element affects the style of the scrollbar of an element.", 
    "standardStatus": "De-facto standard", 
    "ieStatus": { 
      "text": "Under Consideration", 
      "priority": "Medium" 
    }, 
    "ff_views": { 
      "text": "In Development" 
    }, 
    "impl_status_chrome": "Shipped", 
    "uservoiceid": 9081910, 
    "statusid": 333 
  }

[alrra]

Thanks, @FremyCompany!

[alrra]

The second part of the PR is however accurate, we could merge that:

Done! Thanks, @ExE-Boss!

[SelenIT]

@FremyCompany, are you sure that it’s correct to say that WebKit scrollbars styling is in development in Firefox? I understand the current Firefox status as they are implementing the newly proposed spec, which is mostly developed by the same people who implement it, instead of the WebKit’s pseudo-element-based approach.

[FremyCompany]

@SelenIT Ah, yes, sorry, I didn't catch that. I'll fix it.