[css-borders-4] Create corner-shape-explainer.md for TAG review

css-borders-4/corner-shape-explainer.md

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+# Shaping Corners
+
+## The user need
+Squircles, and other shaped corners, have been [sought after](https://www.figma.com/blog/desperately-seeking-squircles/) in the web
+for quite a while.
+
+[Native platforms](https://blog.minimal.app/rounded-corners-in-the-apple-ecosystem/) have had different versions of them for a long time,
+and they have been a common design in the design ecosystem, everywhere except for in the web platform.
+
+## The solution
+To allow for these designs, CSS enhances the existing concept of `border-radius`, by allowing the author to specify how convex/concave
+the shape of the corner should be, alongside its radii.
+
+The `corner-shape` property accepts any value between `notch` (fully concave) and `square` (full convex), using the `superellipse()` function.
+Designers can choose the shape of the corner that they fancy, and also animate between different corner shapes.
+
+Borders and shadows are also shaped according to the given `corner-shape`.
+
+## Why "corners"?
+Alternatively to using the concept of corners, there was an option to use free-form shapes as borders.
+This is expressed in a separate proposal (`border-shape`). However, by extending the concept of corners the web platform allows
+this type of shaping to be a progressive enhancement on top of existing `border-radius`, as well as giving a relatively simple tool
+for shaping the corners rather than overloading everything on top of a general purpose shaping power tool.
+
+In addition, when constraining the problem to "corners", we can lift other constraints that exist on general-purpose shapes: for example,
+border styles are renderable because the edge-cases are more manageable.
+
+
+## Self review S&P questionnaire
+
+01.  What information does this feature expose,
+     and for what purposes?
+
+It does not expose information.
+
+3.  Do features in your specification expose the minimum amount of information
+     necessary to implement the intended functionality?
+
+Yes
+
+5.  Do the features in your specification expose personal information,
+     personally-identifiable information (PII), or information derived from
+     either?
+
+No
+
+6.  How do the features in your specification deal with sensitive information?
+
+No sensitive information
+
+8.  Does data exposed by your specification carry related but distinct
+     information that may not be obvious to users?
+
+No
+
+10.  Do the features in your specification introduce state
+     that persists across browsing sessions?
+
+No
+
+12.  Do the features in your specification expose information about the
+     underlying platform to origins?
+
+No
+
+14.  Does this specification allow an origin to send data to the underlying
+     platform?
+
+No
+
+16.  Do features in this specification enable access to device sensors?
+
+No
+
+18.  Do features in this specification enable new script execution/loading
+     mechanisms?
+
+No
+
+20.  Do features in this specification allow an origin to access other devices?
+
+No
+
+21.  Do features in this specification allow an origin some measure of control over
+     a user agent's native UI?
+
+No
+
+23.  What temporary identifiers do the features in this specification create or
+     expose to the web?
+
+None
+
+25.  How does this specification distinguish between behavior in first-party and
+     third-party contexts?
+
+No
+
+27.  How do the features in this specification work in the context of a browser’s
+     Private Browsing or Incognito mode?
+
+N/A
+
+29.  Does this specification have both "Security Considerations" and "Privacy
+     Considerations" sections?
+
+No
+
+31.  Do features in your specification enable origins to downgrade default
+     security protections?
+
+No
+
+33.  What happens when a document that uses your feature is kept alive in BFCache
+     (instead of getting destroyed) after navigation, and potentially gets reused
+     on future navigations back to the document?
+
+Nothing in particular
+
+35.  What happens when a document that uses your feature gets disconnected?
+
+N/A
+
+37.  Does your spec define when and how new kinds of errors should be raised?
+
+No
+
+39.  Does your feature allow sites to learn about the user's use of assistive technology?
+
+No
+
+40.  What should this questionnaire have asked?
+
+Nothing in addition