Auto merge of #18758 - stuartnelson3:stn/dynamic-body-bgcolor, r=emilio

Add support for dynamic bgcolor change

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes

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tests/wpt/metadata/MANIFEST.json

@@ -168233,6 +168233,18 @@
      {}
     ]
    ],
+   "html/rendering/the-css-user-agent-style-sheet-and-presentational-hints/body-bgcolor-attribute-change.html": [
+    [
+     "/html/rendering/the-css-user-agent-style-sheet-and-presentational-hints/body-bgcolor-attribute-change.html",
+     [
+      [
+       "/html/rendering/the-css-user-agent-style-sheet-and-presentational-hints/body-bgcolor-attribute-change-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "html/semantics/document-metadata/the-link-element/stylesheet-change-href.html": [
     [
      "/html/semantics/document-metadata/the-link-element/stylesheet-change-href.html",
@@ -293042,6 +293054,11 @@
      {}
     ]
    ],
+   "html/rendering/the-css-user-agent-style-sheet-and-presentational-hints/body-bgcolor-attribute-change-ref.html": [
+    [
+     {}
+    ]
+   ],
    "html/rendering/unstyled-xml-documents/.gitkeep": [
     [
      {}
@@ -584743,6 +584760,14 @@
    "da39a3ee5e6b4b0d3255bfef95601890afd80709",
    "support"
   ],
+  "html/rendering/the-css-user-agent-style-sheet-and-presentational-hints/body-bgcolor-attribute-change-ref.html": [
+   "8040cdc2d5096bbb27ebed7539706574faa8515f",
+   "support"
+  ],
+  "html/rendering/the-css-user-agent-style-sheet-and-presentational-hints/body-bgcolor-attribute-change.html": [
+   "3cd7418ba8ea44bc2a643e63d38574c39041de24",
+   "reftest"
+  ],
   "html/rendering/unstyled-xml-documents/.gitkeep": [
    "da39a3ee5e6b4b0d3255bfef95601890afd80709",
    "support"