That an interface has a legacy caller means that an instance can be called as a function. This used to be the case for HTMLEmbedElement and HTMLObjectElement but is being removed. Calling will instead throw an exception. This was deprecated in Chrome 57.
Docs: https://developer.mozilla.org/en-US/docs/Web/API/HTMLEmbedElement https://developer.mozilla.org/en-US/docs/Web/API/HTMLObjectElement