I think that's exactly the problem I was trying to pass: I don't want to use JS to modify the content of an iframe that's embedded in my site. Why? Because it's dirty.

The advantage of using the 'all' property is that big companies like Twitter or Google could simply provide us with a CSS stylesheet that formats a specific HTML to look like a tweet. Then we (as developers) would be able to manage the download of the tweet server-side. This will lead to the fact that the client has to make much less requests to third-party servers -> faster site.

I may have struggled a bit with formulating my idea behind this whole scenario. I will try to do that better in the future... :)