Create document-policy-explainer.md (#328)

Add a first draft of an explainer for document policies (Thanks @Malvoz, @eeeps, @huchenlei, @sideshowbarker, @foolip, @annevk for review)
w3c · Aug 7, 2019 · 7b32587 · 7b32587
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+# Feature Policy: Document Policies
+
+This is a proposal for an extension to Feature Policy to cover those kinds of
+features which don't involve delegation of permission to trusted origins;
+features which are more about configuring a document, or removing features
+(sandboxing) from a document or a frame.
+
+## Start with Examples!
+
+### Performance guardrails
+
+The simplest example is a site which wants to enforce some performance
+best-practices on their own content. They can do this by serving their HTML
+content with this HTTP header:
+
+```http
+Document-Policy: no-unsized-media, no-document-write,
+                 image-compression;bpp=2, frame-loading;lazy
+```
+
+A document served with this header may embed other content, first- or
+third-party, and that content will not be subject to those restrictions. That
+content may include its own `Document-Policy` header, but the headers do not
+combine in any way.
+
+### Enforcing performance guardrails on embedded content
+
+In this example, the top level document wants to ensure that the content loaded
+into a particular frame uses best practices regarding its images. All images
+should have declared sizes, and should be reasonably compressed. It includes an
+iframe tag like this:
+
+```html
+<iframe src="https://img.example.com/"
+        policy="no-unsized-media,image-compression;bpp=2">
+```
+
+When the document is loaded over the network, the request will look like this
+(abbreviated):
+
+```http
+GET / HTTP/1.1
+Host: img.example.com
+Sec-Required-Document-Policy: no-unsized-media,image-compression;bpp=2
+```
+
+The response will need to have a conforming header in order to be loaded, so a
+document served with this header would work:
+
+```http
+Document-Policy: image-compression;bpp=1.5,no-document-write,no-unsized-media
+```
+
+While either of these headers would cause a network error to be returned
+instead:
+
+```http
+Document-Policy: no-unsized-media,image-compression;bpp=3
+Document-Policy: image-compression;bpp=2
+```
+
+If the document is successfully loaded, any subsequent navigations from that
+frame will also have their requests sent with the same
+`Sec-Required-Document-Policy` header.
+
+### Deeper nesting
+
+As above, if the top-level document uses the policy attribute in an iframe
+element, like this:
+
+```html
+<iframe src="https://img.example.com/" policy="image-compression;bpp=2">
+```
+
+Then the request for the frame will look like this:
+
+```http
+GET / HTTP/1.1
+Host: img.example.com
+Sec-Required-Document-Policy: image-compression;bpp=2
+```
+
+If the framed document then includes its own subframes, they will be sent with
+the same `Sec-Required-Document-Policy` header, as the policy set by the
+top-level document applies to all nested iframes within.
+
+If the framed document then also applies a policy attribute to its iframes, then
+the required policy indicated by those frames will combine with the required
+policy imposed on the document itself.
+
+A document like this:
+
+```html
+<html>
+  <body>
+    <iframe src="https://a.example.com/" policy="no-unsized-media"></iframe>
+    <iframe src="https://b.example.com/"
+            policy="image-compression;bpp=1"></iframe>
+    <iframe src="https://c.example.com/"
+            policy="image-compression;bpp=4"></iframe>
+  </body>
+</html>
+```
+
+Would result in these three (simplified) HTTP requests:
+
+```http
+GET / HTTP/1.1
+Host: a.example.com
+Sec-Required-Document-Policy: image-compression;bpp=2,no-unsized-media
+```
+
+```http
+GET / HTTP/1.1
+Host: b.example.com
+Sec-Required-Document-Policy: image-compression;bpp=1
+```
+
+```http
+GET / HTTP/1.1
+Host: c.example.com
+Sec-Required-Document-Policy: image-compression;bpp=2
+```
+
+(Note that in the last example, the stricter requirements imposed by the
+top-level document subsume the requirements on the nested frame, so the combined
+threshold value is still 'bpp=2'.)
+
+### Sandboxing nested content (Traditional sandbox)
+
+As examples of different ways to sandbox content using a combination of the
+`sandbox` and `policy` attributes, the following could all be used to create a
+sandboxed iframe:
+
+```html
+<iframe sandbox>
+<iframe sandbox="allow-scripts allow-forms">
+<iframe policy="no-scripts"> (This is a non-sandboxed frame with no scripting allowed)
+<iframe sandbox policy="scripts"> (Should this be allowed?)
+<iframe sandbox="allow-scripts" policy="no-scripts"> (This one needs some work)
+<iframe policy="no-same-origin"> (This is a frame with an opaque origin, but no other sandboxing)
+```
+
+None of these result in a `Sec-Required-Document-Policy` header, as all of the
+existing sandbox features are safelisted.
+
+However, new features are not safelisted, and so would result in a header if any
+of them are restricted:
+
+```html
+<iframe policy="no-scripts, no-document-write">
+```
+
+results in a request header:
+
+```http
+Sec-Required-Document-Policy: no-document-write
+```
+
+(Note that `no-scripts` is not present in the header; only those values which
+are required to appear in the response header are listed.)
+
+The sandbox attribute in an iframe tag serves to disable all of the
+currently-defined sandbox features. These can also be toggled individually with
+the policy attribute:
+
+```html
+<iframe policy="no-document-write, no-escape-in-popups">
+```
+
+They can also be set from an HTTP header:
+
+```http
+Require-Document-Policy: no-forms
+```
+
+Would impose the sandboxed forms browsing context flag on all nested content.
+
+### Sandboxing yourself
+
+```http
+Document-Policy: no-popups, no-modals, no-presentation-lock, no-forms,
+                 no-pointer-lock
+```
+
+In this case (like in the first document policy example), the document may embed
+other content without these restrictions.
+
+You can also include both headers, to sandbox yourself as well as all nested
+content:
+
+```http
+Document-Policy: no-popups, no-modals, no-presentation-lock, no-forms, no-pointer-lock
+Require-Document-Policy: no-popups, no-modals, no-presentation-lock, no-forms, no-pointer-lock
+```
+
+## So, how does it work?
+
+`Document-Policy` is an HTTP response header that sets the policy on the
+document which it is served with. You can use it to set things like restrictions
+on image sizes or compression ratios, lazy frame and image loading policies, use
+of synchronous APIs, or use of APIs that fall under iframe sandboxing,
+traditionally.
+
+That header doesn't impose any restrictions on content which the document embeds
+at all, and in the base case, every document is free to set its own policy
+independently.
+
+(In a minute, we'll talk about required policies, though - and not conforming
+to a required policy might impact your page's ability to be loaded, but in
+general, you're free to set your policy any way you like)
+
+Policies controllable with this mechanism include things like document
+configuration, and sandboxing. Each of them can be named with a unique string.
+
+Examples:
+
+* `image-compression`
+* `frame-loading`
+* `vertical-scroll`
+* `popups`
+* `document-write`
+* `scripts`
+
+Some policies are boolean switches, and can be turned off with the syntax
+"`no-<feature-name>`":
+
+* `no-popups`
+* `no-scripts`
+* `no-document-write`
+
+Others take parameters, whose types and names are specific to each policy:
+
+* `image-compression;min-bpp=2`
+* `frame-loading;lazy`
+
+All of the directives can be combined with commas into a single header. In
+Structured-header-speak, this is a parameterized list.
+
+### Requiring a policy in your embedded content
+
+It is possible to enforce that content you embed conforms to a specific policy
+as well. This can be done in two ways: either through a different response
+header (`Require-Document-Policy`), which affects every iframe on your page; or
+by using a "`policy`" attribute on specific frames, which target just those
+frames.
+
+`Require-Document-Policy` doesn't affect the document that it is returned with,
+just the content that it embeds. It can be used in conjunction with the
+`Document-Policy` header, though.
+
+When you set a required document policy, most features will have to be
+acknowledged by the document being embedded, as otherwise it can be possible to
+affect the content on that page in unexpected ways. Any policies which require
+acknowledgement will be advertised with a `Sec-Required-Document-Policy` request
+header.
+
+If the content which is returned doesn't have a `Document-Policy` header which
+is at least as strict as the `Sec-Required-Document-Policy`, then it will not be
+loaded into the frame, and a network error will result instead.
+
+The `Sec-Required-Document-Policy` header will be present for all requests for
+documents in a frame, at every nesting level. If the framed document adds its
+own `Require-Document-Policy` header, or uses the policy attribute on its own
+iframes, then the requirements will be combined in deeper requests, to ensure
+that all content meets all of the requirements being imposed on it.
+
+### Extending the Iframe Sandboxing Mechanism
+
+All existing sandbox features can be defined as document policies as well. The
+iframe `sandbox` attribute will disable all of them by default, but they can be
+turned back on either with the `policy` attribute or the `sandbox` attribute.
+Additionally, they can be disabled individually, even on frames which are not
+otherwise sandboxed.
+
+Any new sandbox-style features can be added through this mechanism, without
+having to add them to the set of features disabled by the sandbox attribute.
+
+### Resetting policies in popups
+
+There is currently a sandbox flag, the
+*sandbox propagates to auxiliary browsing contexts flag*, which controls whether
+the existing sandbox features should be inherited in popups opened from a
+sandboxed frame. This flag can continue to function as it currently does, and
+can be extended to cover the entire document policy. This flag itself, as a
+sandbox flag, can be controlled independently as well.
+
+### More Details: Opting in to being embedded with a policy
+
+Each feature has an associated flag, ***requires-acknowledgement***, which is
+true for all of those features which would be unsafe to impose on nested content
+without that content's approval (and is true by default if not explicitly
+defined).
+
+(Alternately, we can define a central safelist of features which do not require
+acknowledgement; the decision comes down to whether that call is made by the
+feature policy spec or the specs which define the individual features)
+
+As a starting point for discussion, all existing sandbox flags would not require
+acknowledgement, and all other document policies would.
+
+In documents generated from `data:` URLs, and in iframe srcdoc documents, where
+the parent document controls the content of the child explicitly, and there is
+no HTTP network transfer, the acknowledgment will simply be implied.
+
+There is also a ***required document policy*** associated with every browsing
+context.
+
+For an iframe, that required policy is the union<sup>[1](#fn1)</sup> of the
+parent's required document policy, the parent's `Require-Document-Policy`
+header, and the policy attribute of the containing iframe element.
+
+All features in the required policy which require acknowledgement will be
+advertised in a `Sec-Required-Document-Policy` request header.
+
+Example:
+
+```http
+Sec-Required-Document-Policy:
+     no-document-write,no-storage,image-compression;bpp=4
+```
+
+If the response does not have a document policy response header which conforms
+to that request header, it will not be loaded. Instead, a network error will be
+returned instead, similar to CSP.
+
+Note: This does not mean strictly increasing strictness: the following would work:
+
+* Top-doc contains `<iframe policy="image-compression;bpp=4">`
+  * First nested doc uses header `image-compression;bpp=2` (okay)
+    * Second nested doc uses header `image-compression;bpp=3` (looser, still okay)
+
+A document's final policy is the union<sup>[1](#fn1)</sup> of its browsing
+context's required policy and it's own header policy. (If it weren't for
+non-opt-in-required policies, the union wouldn't be required; we could just use
+the header policy as-is).
+
+<a name="fn1">1</a>: Note that "union" isn't exactly the right word for the
+combination operation used here; it's something of a "strictest union", where we
+take the strictest value from both sides, independently for each feature. This
+should be spelled more explicitly still, in a proper spec.