html data src,What is the difference between srcdoc="..." and src="data:text/html,..." in an <iframe...

For example, which is the difference between these:

And, in case they are exactly the same, why did HTML5 add srcdoc attribute?

Edit

Maybe I wasn't clear enough. I am not comparing src with srcdoc, but src using text/html data URI with srcdoc.

Then, if the functionality chart is like this

| src attribute | srcdoc attribute

--------------------------------------------------------------------

URL | Yes | No without using src (*)

HTML content | Yes, using data URI | Yes

why is srcdoc needed?

(*) Note:

It seems srcdoc can be used to load a page by URL (Demo), using a subiframe with srcattribute: