The interface is the agent
Page builders sell a canvas. An agent does not need a canvas. It needs a surface it can read, write, and verify.
Most WordPress page builders are drawing tools. They give a person a canvas, a set of handles, and a panel of controls. That design assumes the person is the one moving things around.
A different assumption
An agent does not drag. It reads a structure, writes a change, and then reads the result back to see if the change landed. Every part of that loop must be addressable. If a value only exists inside a visual control, the agent cannot reach it.
This is why Delta stores markup as plain HTML and styles as plain CSS. There is no proprietary node tree to translate. The agent writes what the browser will receive.
Verification is part of the interface
A write that cannot be checked is a guess. Delta pairs every write with a matching read, so an agent can prove the change is live instead of reporting that it should be. That is the part most tools leave out, and it is the part that decides whether the work can be trusted.