Auto merge of #16881 - mbrubeck:absolute_root, r=stshine

Fix containing block size for absolute root element

The root element doesn't have a containing block, so use the viewport size instead of `self.base.absolute_cb`.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #16248 (github issue number if applicable).
- [x] There are tests for these changes

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -3731,6 +3731,18 @@
      {}
     ]
    ],
+   "css/min_width_percent_root_a.html": [
+    [
+     "/_mozilla/css/min_width_percent_root_a.html",
+     [
+      [
+       "/_mozilla/css/min_width_percent_root_b.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "css/min_width_simple_a.html": [
     [
      "/_mozilla/css/min_width_simple_a.html",
@@ -8371,6 +8383,11 @@
      {}
     ]
    ],
+   "css/min_width_percent_root_b.html": [
+    [
+     {}
+    ]
+   ],
    "css/min_width_simple_b.html": [
     [
      {}
@@ -22660,6 +22677,14 @@
    "e8ee634ab6350ad4c870cb1d0ce4f1a88b285581",
    "support"
   ],
+  "css/min_width_percent_root_a.html": [
+   "e8c23bc78daed932d5b513b972582e21711ea06a",
+   "reftest"
+  ],
+  "css/min_width_percent_root_b.html": [
+   "4c5b54f6b4e1bb77a15fdf7d99f9be3a47df7e8b",
+   "support"
+  ],
   "css/min_width_simple_a.html": [
    "e709d1ab4096401ecb7b848d9297c00ef7786bf9",
    "reftest"