In other words, a way to tell Blazor that a given event handler is part of a two-way binding even if you're not using @bind.

It would probably look like this:

<input value="@MyValue" @onchange="MyHandler" @onchange:enforceConsistency />

... or maybe @onchange:sync or @onchange:twoWay. The implementation is simply to emit a builder.SetUpdatesAttributeName call for whatever is the "value" attribute for this element type (e.g., on <input type=checkbox> it would call builder.SetUpdatesAttributeName("checked");).

For more details about the scenario and justification for this design, see #17099 (comment)