LibWeb: Block layout should resolve relative lengths against each box

We were incorrectly resolving relative length units (ex, em, etc.)
against the containing block in many cases. Fix this to resolve them
against the descendant box we're currently processing.

Libraries/LibWeb/Layout/BlockFormattingContext.cpp

@@ -449,12 +449,12 @@ void BlockFormattingContext::place_block_level_replaced_element_in_normal_flow(B
     replaced_element_box_model.padding.top = box.style().padding().top.resolved_or_zero(context_box(), containing_block.width());
     replaced_element_box_model.padding.bottom = box.style().padding().bottom.resolved_or_zero(context_box(), containing_block.width());
 
-    float x = replaced_element_box_model.margin.left.to_px(context_box())
-        + replaced_element_box_model.border.left.to_px(context_box())
-        + replaced_element_box_model.padding.left.to_px(context_box())
-        + replaced_element_box_model.offset.left.to_px(context_box());
+    float x = replaced_element_box_model.margin.left.to_px(box)
+        + replaced_element_box_model.border.left.to_px(box)
+        + replaced_element_box_model.padding.left.to_px(box)
+        + replaced_element_box_model.offset.left.to_px(box);
 
-    float y = replaced_element_box_model.margin_box(context_box()).top + context_box().box_model().offset.top.to_px(context_box());
+    float y = replaced_element_box_model.margin_box(box).top + context_box().box_model().offset.top.to_px(box);
 
     box.set_offset(x, y);
 }
@@ -473,17 +473,17 @@ void BlockFormattingContext::place_block_level_non_replaced_element_in_normal_fl
     box_model.padding.top = style.padding().top.resolved(zero_value, containing_block, containing_block.width());
     box_model.padding.bottom = style.padding().bottom.resolved(zero_value, containing_block, containing_block.width());
 
-    float x = box_model.margin.left.to_px(containing_block)
-        + box_model.border.left.to_px(containing_block)
-        + box_model.padding.left.to_px(containing_block)
-        + box_model.offset.left.to_px(containing_block);
+    float x = box_model.margin.left.to_px(box)
+        + box_model.border.left.to_px(box)
+        + box_model.padding.left.to_px(box)
+        + box_model.offset.left.to_px(box);
 
     if (containing_block.style().text_align() == CSS::TextAlign::VendorSpecificCenter) {
         x = (containing_block.width() / 2) - box.width() / 2;
     }
 
-    float y = box_model.margin_box(containing_block).top
-        + box_model.offset.top.to_px(containing_block);
+    float y = box_model.margin_box(box).top
+        + box_model.offset.top.to_px(box);
 
     // NOTE: Empty (0-height) preceding siblings have their margins collapsed with *their* preceding sibling, etc.
     float collapsed_bottom_margin_of_preceding_siblings = 0;
@@ -502,7 +502,7 @@ void BlockFormattingContext::place_block_level_non_replaced_element_in_normal_fl
         y += relevant_sibling->effective_offset().y() + relevant_sibling->height() + relevant_sibling->box_model().padding.bottom.to_px(*relevant_sibling);
 
         // Collapse top margin with bottom margin of preceding siblings if needed
-        float my_margin_top = box_model.margin.top.to_px(containing_block);
+        float my_margin_top = box_model.margin.top.to_px(box);
 
         if (my_margin_top < 0 || collapsed_bottom_margin_of_preceding_siblings < 0) {
             // Negative margins present.