`data_whtml_builder` silently creates duplicate styles, corrupting the Designer canvas while published output stays correct

[susflow]

Summary

A single data_whtml_builder call that inserts two bare <span> elements into a Rich Text created 59 duplicate styles site-wide. The published site remained pixel-correct, but the Designer canvas and Preview were broken on every page — all grid layouts collapsed. Because the published output was unaffected, the corruption was invisible to every normal verification step.

Environment

• Webflow MCP Server (via Claude Code, webflow-skills plugin v1.0.6)
• Site built on the Lumos framework (container-query based responsive system)
• Date observed: 2026-07-08

Steps to reproduce

1. Open a site whose classes already exist (e.g. a Lumos/Client-First template with u-container, u-grid, u-layout, u-section …).
2. Call data_whtml_builder targeting a Rich Text paragraph inside a component instance's rich-text prop override:

{
  "build_label": "insert span",
  "parent_element_id": { "component": "<pageId>", "element": "<paragraphId>" },
  "creation_position": "prepend",
  "html": "<span>hello</span>"
}

No css parameter is passed. The HTML is a single root element and contains no <style> tag, i.e. it satisfies every rule the webflow_guide_tool documents.
3. Observe the returned element: {"type":"Span","settings":{"visibility":true}} — no styleNames, nothing suspicious.
4. Check Site settings → Backups. The style count has jumped.

Actual result

• Style count went from 1077 → 1136 (+59), per Webflow's own Backups dialog.
• The new styles are -2 / -3 suffixed clones of the existing class names, bound to zero elements:

u-container-2   u-grid-2   u-grid-wrapper-2   u-layout-2   u-layout-wrapper-2
u-layout-column-1-2   u-layout-column-2-2   u-section-2   u-section-spacer-2
u-heading-2   u-text-2   u-button-wrapper-2   u-eyebrow-layout-2   guide_layout-2
card_primary_*-2   button_main_*-2   clickable_*-2 …

That set is precisely the layout/grid style dependency stack of the insertion context.

• Designer canvas and Preview broke on every page of the site, including pages that contain neither the inserted element nor the component it lives in.
• Published output stayed completely correct — the orphan rules are emitted into the stylesheet but applied to no element.
• Deleting the inserted <span> elements did not fix the Designer. The styles persist independently of the elements.
• Restoring a backup taken before the call fixed it immediately.

Expected result

Inserting a classless <span> should create zero styles. At minimum, the tool should not clone existing class definitions under -2 / -3 names, and any style creation should be reported in the tool response.

Why this is severe

1. Silent. The tool response gives no hint that 59 styles were created.
2. Global. A page-scoped call mutates the site-wide style registry.
3. Invisible to normal verification. Checking the published HTML/CSS — the obvious way to confirm a change worked — shows a perfectly healthy site. Only the Designer is broken.
4. Not reversible by undoing the element. Removing the inserted node leaves the styles behind.
5. Undocumented. webflow_guide_tool documents the tool's HTML/CSS rules (single root element, no <style> tags, allowed media queries) but never mentions that it creates styles.

Suspected mechanism

data_whtml_builder appears to serialize the insertion point's style context into a WHTML clipboard payload. Webflow's paste importer then encounters class names that already exist and creates deduplicated copies.

Why the Designer breaks but publishing does not is a hypothesis, not a verified claim: this site's entire responsive system consists of @container rules that live only in a Global Styles HTML embed, keyed off container-type: inline-size carried by .u-grid-wrapper:where(.w-variant-…). The compiled stylesheet contains zero @container rules. If the canvas resolves u-grid-wrapper to the duplicate style — which carries no container-type — no container query ever matches, and every grid falls back to desktop defaults.

Suggested fixes

• Do not create styles for elements that carry no class.
• Never silently clone an existing class under a -2 name; either reuse the existing style or fail loudly.
• Return the list of created styles in the tool response.
• Document the style side effect in webflow_guide_tool, and state that the tool is intended for greenfield structure, not for edits inside existing styled contexts.