Parse poster url relative to the node's document

tests/wpt/mozilla/meta/MANIFEST.json

@@ -32838,7 +32838,7 @@
    "testharness"
   ], 
   "mozilla/video_poster_frame.html": [
-   "2f4af97720c40fbbde8d1cad660b1018d88367e5", 
+   "8b321ef5d82e0de6e16625ecd41190abc00bacb6",
    "reftest"
   ], 
   "mozilla/video_poster_frame_ref.html": [

tests/wpt/mozilla/tests/mozilla/video_poster_frame.html

@@ -13,7 +13,7 @@
         // Apart from removing the `reftest-wait` class we need to get
         // the `load` event to signal that it's ok to take the reftest
         // screenshot. Video loading delays the document `load` event,
-        // so we garantee that we get this event by setting an invaid
+        // so we garantee that we get this event by setting an invalid
         // src attribute to the video tag.
         document.documentElement.classList.remove("reftest-wait");
       });