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It used to be that the default url [the one in the src="" attribute of the iframe] was not treated as a link. Only straight HTML links with target="iframeName" were passing PR, anchor text influence and so on. So the original document loaded in the iframe also needed a straight html link from somewhere.
This may have changed recently on Google but I wouldn't count on a parallel situation for all search engines. I suggest you have a regular link to every potentially iframed document.
Given the above, there are things you can do to make sure that your <iframe> content is accessible to both search engines and visitors.
16.5 Inline frames: the IFRAME element [w3.org]
The information to be inserted inline is designated by the src attribute of this element. The contents of the IFRAME element, on the other hand, should only be displayed by user agents that do not support frames or are configured not to display frames.
The content of the <iframe> element would be that which resides between the opening and closing tags for the element...
<iframe>The content for user agents that do not support frames or are configured not to display frames. Add a link here to the src="" of the <iframe> element.</iframe> And yes, a link to the
src="" of the <iframe> element is in order when optmizing your content within the <iframe></frame> tags. P.S. You can also use the title attribute for the <iframe>...
<iframe title="Appropriate title for the <iframe> element goes here."></iframe>
As stated above, the guidelines tell you how to effectively optimize the <iframe> element for maximum accessibility. Here is how I've handled it since implementing <iframes> a few years ago. papabaer (a famous CSS guru), turned me on to <iframes> and how to use them creatively.
If you are using an <iframe> that contains content from an offsite source (not within your domain), you should use a short description of that resource along with a link to src of the <iframe>. For example...
<iframe src="" title="">Enter a short description of the linked resource along with a link to that source in this area.</iframe>
Now, there is no limit to the amount of content you can place between that opening and closing <iframe> element. If I were displaying a page from within the same domain, I would take everything on that page and place it between the opening and closing <iframe> elements. This way those users who have frames disabled will see what they should have seen had they not disabled their frames capabitlities. Keep in mind, this also applies to indexing spiders. So, it is a win/win situation. The end user gets what they should get and the spiders get what they should get.
Design for the user first, the spiders will naturally follow. It took me a few years for that to sink in (way back when), but once it sinks in, everything becomes habit. ;)
P.S. There is a standard practice for identifying content that is to display if certain features do not work. You'll notice that many times, information is surround by [Infomation Here] brackets. Those brackets have meaning to various technologies.