[tor-bugs] #21793 [Applications/Tor Browser]: Keep an eye on the CustomElementRegistry API

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#21793: Keep an eye on the CustomElementRegistry API
-------------------------------------+-------------------------------------
     Reporter:  gk                   |      Owner:  tbb-team
         Type:  task                 |     Status:  new
     Priority:  Medium               |  Milestone:
    Component:  Applications/Tor     |    Version:
  Browser                            |   Keywords:  ff52-esr, tbb-
     Severity:  Normal               |  linkability
Actual Points:                       |  Parent ID:
       Points:                       |   Reviewer:
      Sponsor:                       |
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 Custom Elements (https://developer.mozilla.org/en-
 US/docs/Web/Web_Components/Custom_Elements) can get registered,
 manipulated and queried by web content. We should make sure there are no
 linkability risks are associated with that.

 For ESR 52 this seems to be still disabled by
 `dom.webcomponents.customelements.enabled = false`. We should check that,
 though.

 The tracking bug is https://bugzilla.mozilla.org/show_bug.cgi?id=889230.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21793>
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