Remove rel attribute from a element - fixes #344 (#346)

Author: mozfreddyb

builtins/safe-default-configuration.json

@@ -254,10 +254,6 @@
           "name": "hreflang",
           "namespace": null
         },
-        {
-          "name": "rel",
-          "namespace": null
-        },
         {
           "name": "type",
           "namespace": null

builtins/safe-default-configuration.txt

@@ -60,13 +60,13 @@ div
 
 a
 - href
-- rel
 - hreflang
 - type
 // Purposely omitted:
 // - target
 // - download
 // - referrerpolicy
+// - rel
 // - ping
 em
 strong

index.bs

@@ -1550,6 +1550,6 @@ A more complete treatment of mXSS can be found in [[MXSS]].
 
 This work is informed and inspired by [[DOMPURIFY]] from cure53,
 Internet Explorer's {{window.toStaticHTML()}} as well as the original
-[[HTMLSanitizer]] from Ben Bucksch.
-Anne van Kesteren, Krzysztof Kotowicz, Tom Schuster, Luke Warlow,
-Guillaume Weghsteen, and Mike West for their valuable feedback.
+[[HTMLSanitizer]] from Ben Bucksch. Anne van Kesteren, Krzysztof Kotowicz,
+Andrew C. H. Mc Millan, Tom Schuster, Luke Warlow, Guillaume Weghsteen,
+and Mike West for their valuable feedback.