An HTML element is described as replaced, if its content cannot be styled by CSS because it is rendered by a mechanism that is external to CSS.
E.g. A browser plugin (applet, embed & object), the Shadow DOM (audio & video), Javascript (canvas), a nested browser window (iframe), or browser internal (img).
A replaced element is replaced with content that is outside of the scope of CSS.
w3.org | Replaced element
An element whose content is outside the scope of the CSS formatting model, such as an image, embedded document, or applet.
The content of replaced elements is not considered in the CSS rendering model.
The following HTML elements are replaced:
See also: