This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
<multicol> element was an experimental element designed to allow multi-column layouts. It never got any significant traction and is not implemented in any major browsers.
Note that up to Firefox 22, though not supported, a
<multicol> element was associated with an
HTMLSpanElement. It was then fixed and is now associationd with a
HTMLUnknownElement as requested by the specification.