Auto merge of #13401 - notriddle:master, r=pcwalton

Implement sequential fallback to float speculation

This shouldn't impact any pages that are already rendering correctly, but it is a very naive implementation of this pass.

---

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #13284 and fix #13223
- [X] There are tests for these changes

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components/layout/inline.rs

@@ -1482,7 +1482,10 @@ impl Flow for InlineFlow {
                     flow::base(kid).flags.is_float() {
                 continue
             }
-            kid.assign_block_size_for_inorder_child_if_necessary(layout_context, thread_id);
+            let content_box = flow::base(kid).position;
+            kid.assign_block_size_for_inorder_child_if_necessary(layout_context,
+                                                                 thread_id,
+                                                                 content_box);
         }
 
         if self.contains_positioned_fragments() {