What's the valid way to include an image with no src?
While there is no valid way to omit an image's source, there are sources which won't cause server hits. I recently had a similar issue with iframes and determined //:0 to be the best option. No, really!
Starting with // (omitting the protocol) causes the protocol of the current page to be used, preventing "insecure content" warnings in HTTPS pages. Skipping the host name isn't necessary, but makes it shorter. Finally, a port of :0 ensures that a server request can't be made (it isn't a valid port, according to the spec).
This is the only URL which I found caused no server hits or error messages in any browser. The usual choice — javascript:void(0) — will cause an "insecure content" warning in IE7 if used on a page served via HTTPS. Any other port caused an attempted server connection, even for invalid addresses. (Some browsers would simply make the invalid request and wait for them to time out.)
This was tested in Chrome, Safari 5, FF 3.6, and IE 6/7/8, but I would expect it to work in any browser, as it should be the network layer which kills any attempted request.
Another option is to embed a blank image. Any image that suits your purpose will do, but the following example encodes a GIF that is only 26 bytes - from http://probablyprogramming.com/2009/03/15/the-tiniest-gif-ever
<img src="data:image/gif;base64,R0lGODlhAQABAAD/ACwAAAAAAQABAAACADs=" width="0" height="0" alt="" />
Edit based on comment below:
Of course, you must consider your browser support requirements. No support for IE7 or less is notable. http://caniuse.com/datauri
These days IMHO the best short, sane & valid way for an empty img src is like this:
<img src="data:," alt>
or
<img src="data:," alt="Alternative Text">
The second example displays "Alternative Text" (plus broken-image-icon in Chrome and IE).
"data:," is a valid URI. An empty media-type defaults to text/plain. So it represents an empty text file and is equivalent to "data:text/plain,"
OT: All browsers understand plain
alt. You can omit ="" , it's implicit per HTML spec.