Store harmonized border-{...}-width in computed value collapse_border…

…_{...}_width

weasyprint/formatting_structure/build.py

@@ -1025,18 +1025,9 @@ def collapse_table_borders(table, grid_width, grid_height):
     horizontal_borders = [[weak_null_border for x in range(grid_width)]
                           for y in range(grid_height + 1)]
 
-    def backup_border_width(box_style, side):
-        box_style[f'__border_{side}_width'] = box_style[f'border_{side}_width']
-
-    def restore_border_width(box_style, side):
-        if f'__border_{side}_width' in box_style:
-            box_style[f'border_{side}_width'] = \
-                box_style[f'__border_{side}_width']
-
     def set_one_border(border_grid, box_style, side, grid_x, grid_y):
         from ..draw import get_color
 
-        restore_border_width(box_style, side)
         style = box_style[f'border_{side}_style']
         width = box_style[f'border_{side}_width']
         color = get_color(box_style, f'border_{side}_color')
@@ -1107,8 +1098,7 @@ def set_borders(box, x, y, w, h):
     # the correct widths on each box. The actual border grid will be
     # painted separately.
     def set_transparent_border(box, side, twice_width):
-        backup_border_width(box.style, side)
-        box.style[f'border_{side}_width'] = twice_width / 2
+        box.style[f'collapse_border_{side}_width'] = twice_width / 2
 
     def remove_borders(box):
         set_transparent_border(box, 'top', 0)