The seamless attribute is used to embed an <iframe> in the calling page without scrollbars or borders (e.g. seamlessly) and with the parent document's styles applying to the embedded seamless iframe.
Craig Francis commented
It looks like the iframe[seamless] attribute has been removed from the WHATWG spec:
This is due to it being too complicated.
Instead I'm proposing a solution to simply have the iframe resize its height (to avoid scroll bars), which seems to be the main requirement from developers: