Add support for <video preload=...>, <audio preload=...>

This commit adds "preload" to the set of acceptable attributes on video
and audio elements.

Author: wingo

lib/html5/filters/validator.rb

@@ -167,8 +167,8 @@ class HTMLConformanceChecker < HTML5::Filters::Base
     # <embed> handled separately
     'object'       => %w[data type usemap height width],
     'param'        => %w[name value],
-    'video'        => %w[src autoplay start loopstart loopend end loopcount controls poster],
-    'audio'        => %w[src autoplay start loopstart loopend end loopcount controls],
+    'video'        => %w[src autoplay start loopstart loopend end loopcount controls poster preload],
+    'audio'        => %w[src autoplay start loopstart loopend end loopcount controls preload],
     'source'       => %w[src type media],
     'canvas'       => %w[height width],
     'map'          => [],

lib/html5/sanitizer.rb

@@ -45,9 +45,9 @@ module HTMLSanitizeModule
       dir disabled enctype for frame headers height href hreflang hspace id
       ismap label lang longdesc loop loopcount loopend loopstart
       maxlength media method multiple name nohref
-      noshade nowrap poster prompt readonly rel rev rows rowspan rules scope
-      selected shape size span src start style summary tabindex target title
-      type usemap valign value vspace width xml:lang]
+      noshade nowrap poster preload prompt readonly rel rev rows rowspan rules
+      scope selected shape size span src start style summary tabindex target
+      title type usemap valign value vspace width xml:lang]
 
     MATHML_ATTRIBUTES = Set.new %w[actiontype align close
       columnalign columnlines columnspacing columnspan depth display