Non-standard: 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.
unsized-media directive controls whether the current document is allowed to change the size of media elements after the initial layout is complete.
This restriction solves "layout instability" problem caused by providing default dimensions for images whose size is not specified in advance so that image doesn't change size after loading.