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.
Isaque Neves commented
is fantastic to improve the Microsoft Edge
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: