https://drafts.csswg.org/css-multicol-1/#example-c0153f49

The red border in figure 11 looks wrong to me. I think only the first fragment should have a block-start border and only the last fragment should have a block-end border. Intermediary fragments should not have block-start/end borders. (All fragments should have inline-start/end borders though.) I'm assuming box-decoration-break has its initial value in this example. box-decoration-break:clone should make all fragments have borders on all sides, as usual.