These are old webkit-specific properties that allowed developers to fragment content over columns inside a scrollable region. See: https://bl.ocks.org/grorg/99eb053b432070b05d1e29759d81458f Practically speaking this isn't used. Most developers use it accidentally, and typically when it is used this property to forces a new formatting context similar to setting "overflow: hidden".
For developers who wish to have fragmented content, a more standardized way to do this is with multi-column: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Columns See the previous example using this technique: https://jsbin.com/vovelabewe/edit?html,output For developers wishing to establish a new formatting context (to prevent margin-collapsing or similar), display: flow-root, or overflow: hidden both work. This feature isn't widely used, and maintaining it indefinitely will likely result it a maintenance burden.