Add privacy/security doc for unsanitized HTML. (#699)

* Add privacy/security doc for unsanitized HTML.

* Address PR comments.

ClipboardAPI/tag-security-privacy-clipboard-unsanitized-read.md

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+### Questions from https://www.w3.org/TR/security-privacy-questionnaire/
+
+## 2. Questions to Consider
+
+### 2.1. What information might this feature expose to Web sites or other parties, and for what purposes is that exposure necessary?
+
+This feature exposes unsanitized HTML to the clipboard using the async clipboard API. This information is already exposed by the existing DataTransfer API, so it's not something new that we're exposing with this feature.
+Sites have to opt into reading the unsanitized HTML content, else, they will get sanitized HTML by-default. To read unsanitized HTML via the async clipboard API, all the restrictions related to secure context, permission etc apply.
+
+### 2.2. Do features in your specification expose the minimum amount of information necessary to enable their intended uses?
+
+Yes.
+
+### 2.3. How do the features in your specification deal with personal information, personally-identifiable information (PII), or information derived from them?
+
+This API doesn't expose any new information that can't already be accessed via existing DataTransfer clipboard APIs.
+
+### 2.4. How do the features in your specification deal with sensitive information?
+
+This feature doesn't deal with any sensitive information.
+
+### 2.5. Do the features in your specification introduce new state for an origin that persists across browsing sessions?
+
+No.
+
+### 2.6. Do the features in your specification expose information about the underlying platform to origins?
+
+No.
+
+### 2.7. Does this specification allow an origin to send data to the underlying platform?
+
+No.
+
+### 2.8. Do features in this specification enable access to device sensors?
+
+No.
+
+### 2.9. Do features in this specification enable new script execution/loading mechanisms?
+
+No.
+
+### 2.10. Do features in this specification allow an origin to access other devices?
+
+No.
+
+### 2.11. Do features in this specification allow an origin some measure of control over a user agent’s native UI?
+
+No.
+
+### 2.12. What temporary identifiers do the features in this specification create or expose to the web?
+
+None.
+
+### 2.13. How does this specification distinguish between behavior in first-party and third-party contexts?
+
+It doesn't distinguish between behavior in first-party and third-party contexts as the async clipboard APIs already have restrictions via Permissions.
+
+### 2.14. How do the features in this specification work in the context of a browser’s Private Browsing or Incognito mode?
+
+It works the same way in incognito mode.
+
+### 2.15. Does this specification have both "Security Considerations" and "Privacy Considerations" sections?
+
+Yes. https://github.com/w3c/editing/blob/gh-pages/docs/clipboard-unsanitized/explainer.md#privacy-and-security.
+
+### 2.16. Do features in your specification enable origins to downgrade default security protections?
+
+No.
+
+### 2.17. How does your feature handle non-"fully active" documents?
+
+No interaction with documents regardless of its state. Web authors have to provide the HTML content via the async clipboard APIs.
+
+### 2.18. What should this questionnaire have asked?
+
+N/A
+
+## 3. Threat Models
+
+### 3.1. Passive Network Attackers
+
+No threat.
+
+### 3.2. Active Network Attackers
+
+The API is only available from a secure context.
+
+### 3.3. Same-Origin Policy Violations
+
+It doesn't leak data across origins.
+
+### 3.4. Third-Party Tracking
+
+No interaction with third-party pages.
+
+### 3.5. Legitimate Misuse
+
+We don't think there is any risk of legitimate misuse of this API. Browser parses the HTML string provided by the web author and creates a well-formed HTML
+document before it writes to the system clipboard. The target apps that reads this data will not be at risk of parsing malformed HTML.