Denying consent is better if undetectable as such (#476)

* Denying consent is better if undetectable as such

This is not a full generalization of the concepts that discussed in #470 and #475, but I think that it suffices.

Closes #475.

* Link feature detection to consent

This is based on @dbaron's excellent feedback.  However, I took an extra
step with the last sentence here, which I'm not committed to.  There's
an argument to be had that anything like this probably shouldn't be part
of the web platform.

Now, with the controversy established, go!

* Typu

* can-be

Co-authored-by: Amy Guy <[email protected]>

---------

Co-authored-by: Amy Guy <[email protected]>

index.bs

@@ -218,6 +218,11 @@ of a new feature must justify the additional burden on users
 to decide whether to grant permission for each feature
 whenever it's requested by a Web page.
 
+Refusal is most effective if the site cannot
+distinguish refusal from other, common situations.
+This can make it more difficult for a site to
+pressure users to grant consent.
+
 <p class="example">
 For example,
 the [Geolocation API](https://www.w3.org/TR/geolocation-API/)
@@ -558,11 +563,19 @@ in the language where it's used
 In some cases, it may **not** be appropriate to allow feature detection.
 Whether the feature should be detectable or not
 should be based on the <a href="#priority-of-constituencies">user need</a>
-for the feature:
-if there is a user need or design principle which would fail
+for the feature.
+If there is a user need or design principle which would fail
 if feature detection were available for the feature,
 then you should not support feature detection.
 
+Detecting the availability of a feature does not imply
+detecting whether <a href="#consent">consent</a> to use the feature
+has been granted.
+Generally, detecting whether the feature is implemented
+can be done separately from determining whether use of the feature has been authorized.
+In some cases, it might be necessary to disable feature detection
+in order to enable denying requests to use the feature.
+
 Also, if a feature is generally not exposed to developers,
 it is not appropriate to support feature detection.
 For example, private browsing mode is a concept
@@ -576,6 +589,7 @@ See also:
 * [[#do-not-expose-use-of-private-browsing-mode]]
 * [[#do-not-expose-use-of-assistive-tech]]
 * [[#secure-context]]
+* [[#consent]]
 
 <h3 id="secure-context">Consider limiting new features to secure contexts</h3>