diff --git a/weasyprint/layout/percent.py b/weasyprint/layout/percent.py
index f60e50a1..c2f405f0 100644
--- a/weasyprint/layout/percent.py
+++ b/weasyprint/layout/percent.py
@@ -96,9 +96,9 @@ def resolve_percentages(box, containing_block, main_flex_direction=None):
     # Used value == computed value
     for side in ('top', 'right', 'bottom', 'left'):
         prop = f'border_{side}_width'
-        if collapse and f'collapse_{prop}' in box.style:
-            setattr(box, prop, box.style[f'collapse_{prop}'])
-        else:
+        # border-{side}-width would have been resolved
+        # during border conflict resolution for collapsed-borders
+        if not (collapse and hasattr(box, prop)):
             setattr(box, prop, box.style[prop])
 
     # Shrink *content* widths and heights according to box-sizing
diff --git a/weasyprint/layout/preferred.py b/weasyprint/layout/preferred.py
index 34ca1f7a..58d1a76f 100644
--- a/weasyprint/layout/preferred.py
+++ b/weasyprint/layout/preferred.py
@@ -149,13 +149,13 @@ def margin_width(box, width, left=True, right=True):
 
     collapse = box.style['border_collapse'] == 'collapse'
     if left:
-        if collapse and 'collapse_border_left_width' in box.style:
-            width += box.style['collapse_border_left_width']
+        if collapse and hasattr(box, 'border_left_width'):
+            width += box.border_left_width
         else:
             width += box.style['border_left_width']
     if right:
-        if collapse and 'collapse_border_right_width' in box.style:
-            width += box.style['collapse_border_right_width']
+        if collapse and hasattr(box, 'border_right_width'):
+            width += box.border_right_width
         else:
             width += box.style['border_right_width']
 
diff --git a/weasyprint/layout/table.py b/weasyprint/layout/table.py
index 7e25a730..abadbf90 100644
--- a/weasyprint/layout/table.py
+++ b/weasyprint/layout/table.py
@@ -1080,7 +1080,8 @@ def collapse_table_borders(table):
     # the correct widths on each box. The actual border grid will be
     # painted separately.
     def set_transparent_border(box, side, twice_width):
-        box.style[f'collapse_border_{side}_width'] = twice_width / 2
+        prop = f'border_{side}_width'
+        setattr(box, prop, twice_width / 2)
 
     def remove_borders(box):
         set_transparent_border(box, 'top', 0)