HTML5 forms with base.href

I'm working on a REST API, and to keep things simple for myself I'm using text/html for my primary media type.  The big advantage is that I can use a general purpose web browser to test my API by hand.

In this particular case, the client communicates with the API via a services of web form submissions.  That allows my server to control which cached representations will be invalidated when the form is submitted.

As an experiment, instead of copying the target-uri into the form actions, I decided to try using a BASE element, with no action at all, expecting that the form would be submitted to the base href.

But what the HTML 5 specification says is:

If action is the empty string, let action be the URL of the form document.

So that doesn't work.  Via stack overflow, I discovered a candidate work around - the target of the form can be a fragment; the fragment delimiter itself means the form action is not empty, and therefore relative lookup via the base element is enabled.

Somewhat worrisome: I haven't managed to find clear evidence that form behavior with fragments is well defined.  It occurred to me that perhaps  the standard was playing clever with URL vs URI, but I abandoned that idea after clicking a few more links to review the examples of URLs, and discovered fragments there.

I suspect that I need to let go of this idea - it's not that much extra work to copy the base uri into the forms that want it, and I've got a much clearer picture of what happens in that case.