Updated: October 2016

The seamless attribute no longer exists. It was originally pitched to be included in the first HTML5 spec, but subsequently dropped. An unrelated attribute of the same name made a brief cameo in the HTML5.1 draft, but that too was ditched mid-2016:

So I think the gist of it all both from the implementor side and the web-dev side is that seamless as-specced doesn’t seem to be what anybody wanted to begin with. Or at least it’s more than anybody actually wanted. And anyway like @annevk says, it’s seems a lot of it’s since been “overcome by events” in light of Shadow DOM.

In other words: purge the seamless attribute from your memory, and pretend it never existed.

For posterity’s sake, here’s my original answer from five years ago:

Original answer: April 2011

The attribute is in draft mode at the moment. For that reason, none of the current browsers are supporting it yet (as the implementation is subject to change). In the meantime, it’s best just to use CSS to strip the borders/scrollbars from the iframe:

iframe[seamless]{
    background-color: transparent;
    border: 0px none transparent;
    padding: 0px;
    overflow: hidden;
}

There’s more to the seamless attribute than what can be added with CSS: part of the reasoning behind the attribute was to allow nested content to inherit the same styles applied to the iframe (acting as though the embedded document was one big nested inside the element, for example).

Lastly, versions of Internet Explorer (8 and earlier) require additional attributes in order to remove the borders, scrollbars and background colour:

<iframe frameborder="0" allowtransparency="true" scrolling="no" src="https://stackoverflow.com/questions/4804604/..."></iframe>

Naturally, this doesn’t validate. So it’s up to you how to handle it. My (picky) approach would be to sniff the agent string and add the attributes for IE versions earlier than 9.

Hope that helps. 🙂