Collapsed Table Borders: CSS 2.1 §17.6.2 Conflict Resolution

A border-collapse: collapse table resolves exactly one border per shared grid line - the widest declared border wins, ties break by style priority, further ties by how specific the declaring box is (cell > row > row-group > column > column-group > table), and finally by position. Every other participant's own border at that edge is suppressed rather than separately painted.

Width, then origin, decide each edge

A	B
The 8pt double border on this cell's own bottom edge outranks the row's 4pt orange border below - cell beats row regardless of which one is wider here.	Where no cell states an opinion, the row's own 4pt orange border wins instead.

<col>/<colgroup> border and background participation

Column A	Column B
The colgroup's 3pt green border frames both columns; column A's own narrower red border wins the shared edge between them.	Column backgrounds layer under the cells' own white background, per CSS 2.1 §17.5.1's table → column-group → column → row → cell order.

border-style: hidden suppresses an edge outright

Left	Right
No border here	or here - hidden always wins, whatever else competes for this edge

A repeated <thead>'s boundary resolves fresh on every page

Only row 1 - the header's real DOM neighbor - has the bold red top border. On the first page the header sits directly above it, so that border is what's resolved and drawn. On every later page the header repeats above a different row instead, and the boundary is re-resolved against that row - not reused from page 1 - so only the first page shows the red border.

Row	Note
Row 1	The first body row's own border wins here - it's the true DOM neighbor of the header.
Row 2	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 3	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 4	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 5	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 6	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 7	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 8	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 9	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 10	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 11	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 12	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 13	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 14	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 15	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 16	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 17	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 18	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 19	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 20	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 21	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 22	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 23	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.
Row 24	An ordinary row - every later page's header boundary resolves against whichever row actually starts that page, not row 1's.