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Imagine a profile form with a text field, a Save button, and a
working stored-value update.

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The confirmation message never appears, so the user cannot tell
whether the requested action finished.

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A repair could add that message, yet accidentally remove the
already working Save behavior.

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A prettier screenshot would not reveal that regression without
trying the control again.

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This is an original schematic example, with no real user data or
measured repair outcome.

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It introduces WebWorld's central question: what evidence should
allow a proposed web-code repair to survive?

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WebWorld treats the browser as an executable simulator of how an
HTML artifact behaves under user actions.

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The runtime renders the current page, applies interaction
probes, and records evidence of what happened.

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That evidence includes screenshots, document state, console
information, probe outcomes, and an action trace.

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A vision-language model reads those observations and proposes a
critique, rather than certifying its own repair.

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A planner converts the critique into something the browser can
check after the candidate edit.

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Here, world model means the web execution platform; it does not
mean a physical-world prediction model.

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An interaction contract states what must improve, how to test
it, and what existing behavior must survive.

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In our form example, the target is a visible confirmation after
pressing the Save button.

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The replay specifies a stable control, an input action, and the
state to inspect afterward.

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The preserve set carries previously verified predicates, so
fixing one target cannot silently erase prior progress.

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For a local issue, the contract also limits the edit's impact to
the relevant region.

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These are bounded claims about specified interactions, rather
than a promise that every possible action will work.

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A typed repair skill produces a constrained patch inside the
contract's impact scope.

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Candidate filters can discard unrelated, duplicate, or
preservation-risk edits before they reach the browser.

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That narrows the search, but it does not establish that the page
now works.

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The candidate must be executed again, with evidence tied to its
contract and recorded proof level.

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For the interactive form, replay checks the new confirmation and
rechecks the previously working Save behavior.

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If either fails, our illustrative repair should remain a
rejected candidate rather than become the next baseline.

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WebWorld distinguishes safe runtime steps from
certificate-backed quality steps that can be exported for
training.

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A probe run, partial fix, or rejected edit can still help decide
what to try next.

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But those steps do not enter the supervised training pool simply
because the agent was active.

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Only admitted transitions carry the artifact, the failure it
addressed, and the evidence that justified acceptance.

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Previously verified behavior becomes part of later preservation
checks, creating the proposed quality ratchet.

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The learning example therefore describes verified progress
between states,

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rather than only a final preferred screenshot.

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The experiments compare raw HTML supervision

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with supervision selected from thirty-two thousand eight hundred
accepted transitions.

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Within each model size, the backbone family, prompt template,
optimizer, and evaluation runner remain fixed.

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This makes the source of supervision the central difference in
the matched training comparison.

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HTMLBench four hundred is the primary evaluation, with
MiniAppBench validation used to study transfer.

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The construction pipeline reports separation from benchmark
prompts and their deterministic test cases.

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The following chart compares the two
twenty-seven-billion-parameter rows, not unrelated
frontier-model configurations.

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The raw-supervision twenty-seven-billion-parameter row reports
an HTMLBench composite score of forty-seven point four.

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The WebWorld row at the same model size reports fifty-two point
seven under that benchmark.

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That is a gain of five point three score points in this matched
training setup.

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The composite score is distinct from the separately reported
test-case pass percentage.

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The same model-size pair improves the MiniAppBench validation
average by fourteen point nine points.

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These results concern the reported interactive HTML benchmarks,

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rather than a guarantee of correctness for every website.

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A separate nine-billion-parameter ablation keeps critique and
repair routing while loosening the certificate gate.

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Without the certificate, its HTMLBench gain over Raw is only
zero point four score points.

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The full WebWorld admission rule gives four point eight points
over Raw at that model size.

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The experiment therefore supports more than the value of asking
a model to critique its own page.

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It tests whether the loop's selected training transitions are
backed by the specified verification rule.

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The effect size belongs to this matched ablation; it should not
be generalized to every browser-checking system.

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Return to our original form: a confirmation matters, but so does
the Save action that worked before.

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A candidate change should be described as a concrete target,
then tested under the relevant interactions.

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The working parts become explicit preservation obligations,
rather than assumptions hidden behind a polished screenshot.

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WebWorld makes that distinction part of both the repair loop and
the training-data admission rule.

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A proposed improvement is a hypothesis; the recorded contract
evidence determines whether the system should keep it.

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That is the idea to carry forward: self-improvement needs a
definition of progress that survives execution.