Should the implementation of a hybrid element as defined fall through, an alternative is to define 2 elements (<search> and <searchform>) for the 2 use-cases:
<search>===<div role=search><searchform action="...">===<form role=search action="...">
Implementing this is trivial by aliasing HTMLElement -> <search> and HTMLFormElement -> <searchform>. The specification still needs rewording to incorporate <searchform> as a <form>. The only benefit would be that the action attribute's specification is not changed.
Should 2 elements find opposition, the proposal would be to favor the best practice:
<search action="...">===<form role=search action="..."><search>can be used with form submission dissabled by thesubmitevent handler.<div role=search>remains the suggested usage if form functionality is to be avoided.
Implementing this is trivial by aliasing HTMLFormElement -> <search>. he specification still needs rewording to incorporate <search> as a <form>. The drawback is that a few use-cases of the original request is not treated as first-class. The reasoning above implies that the best practice is more important to be treated as first-class, thus if a choice has to be made it should be to favor form semantics. The hybrid element proposal makes a strong effort to avoid such trade-off.