web/core: Add basic float support

Note that the "clear" property is not yet supported.
There are also some obvious differences in rendering
compared to firefox, but those are due to a lack of
understanding of the spec on my part, I'll come back
to them later once i understand why what I'm doing
is not right.

crates/web/core/src/css/layout/box_tree.rs

@@ -64,7 +64,7 @@ impl BoxTree {
             viewport,
         };
 
-        let mut root_formatting_context = BlockFormattingContext::default();
+        let mut root_formatting_context = BlockFormattingContext::new(initial_containing_block);
         let mut state = BlockFlowState::new(
             initial_containing_block,
             length_resolution_context,