The /deep/ selector was a Shadow DOM CSS combinator for styling descendants across a shadow boundary. It addressed cases where component isolation was useful but selected styles needed to cross into a component. section 10.2
Matching inside and outside the shadow boundary
A rule such as `/deep/ .example { color: red; }` could style matching elements whether they appeared inside a Shadow DOM component or in ordinary document markup. Unlike `::shadow`, it was intended to match across the boundary rather than only dive into shadow content. section 10.2
Treat /deep/ as historical rather than deployable CSS. It belonged to Shadow DOM v0, never entered v1, and Chrome removed support in version 63; there is no direct replacement for its functionality. section 10.2
