docs(proposals): web pages the model writes, for the agent and Studio

docs/adr/0066-web-pages-are-html-the-model-writes-without-network.md

@@ -0,0 +1,27 @@
+# ADR 0066: Web pages are HTML the model writes, run in a frame that has no network
+
+- **Status:** Proposed, with the [model-written web pages proposal](../proposals/model-written-web-pages.md)
+- **Date:** 2026-10-21
+- **Supersedes:** for Studio's html format with a model at the agent's bar, and for the agent's web pages, the rule in [ADR 0010](0010-studio-builders-not-sandboxes.md) that a model emits structured content and a builder renders it
+- **Source:** maintainer decision, 21 Oct 2026 (no permalink)
+
+## Context
+
+Studio's html format has the model return plain-text sections, escaped into a fixed template, so a page cannot carry a script ([ADR 0010](0010-studio-builders-not-sandboxes.md), [studio](../architecture/studio.md)). The agent uses the same path through `create_artifact`, so every page is unstyled text however capable the model. Word and PDF already split by the model's measured level: a model at the agent's bar writes a script, one below it Markdown for a builder ([model capabilities](../architecture/model-capabilities.md)). [ADR 0028](0028-model-written-code-runs-with-approval.md) lets model-written code run, and the agent's documents are model-written scripts ([ADR 0039](0039-document-scripts-run-without-approval.md)).
+
+A page is built from the user's sources, which can carry instructions aimed at the model. The viewer's frame has an opaque origin, so a script cannot reach the app, but it can still make requests, open popups and navigate itself, each of which can carry what the page read off the machine.
+
+## Decision
+
+- A web page may be HTML the model writes: any markup, CSS, script and inline SVG. The agent's pages are, at most 5 MB. Studio's are when `writes_script()` says the model writes the file, as for Word and PDF, at most 300 KB; below that, Studio keeps the template.
+- SurfSense stores a page as written, plus one CSP `<meta>`, and runs it only in a sandboxed frame.
+- A page has no network. Its CSP allows only inline script and style and `data:` or `blob:` images, fonts and media; the viewer's frame drops `allow-popups`; Electron refuses a subframe's navigation to another origin. No CDN and no font host is allowed, since a request's URL can carry data. This holds every html artifact, the template's included.
+- One design guide is read by both paths, the agent's skill and Studio's prompt.
+- Whether the template stays for models below the bar is decided by measuring the ladder's models on the HTML prompt.
+
+## Consequences
+
+- Pages can be designed, interactive and animated. The agent's are versioned, screenshotted and limited to three failed saves a turn; Studio's are checked for structure only, three attempts.
+- A page cannot load a library or web font; it inlines what it needs.
+- Links inside a page do not open from the viewer until a later change routes them through the app.
+- A model may write a poor or broken page. The agent sees its screenshots; Studio sees only the structural checks, as its Word scripts do.

docs/adr/README.md

@@ -93,3 +93,4 @@ The Date is the day the decision was first written down. Source links are permal
 | 0063 | [A plugin's connection is a transport and one sign-in, detected from the server](0063-a-plugins-connection-is-a-transport-and-one-sign-in.md) | Accepted |
 | 0064 | [Agent runtimes reach plugins only through SurfSense's gateway, and never hold a plugin's credential](0064-agent-runtimes-reach-plugins-through-surfsenses-gateway.md) | Accepted |
 | 0065 | [A turn's plugin tools are sized by the model's window, not counted, and Claude Code searches its own](0065-a-turns-plugin-tools-are-sized-by-the-window-not-counted.md) | Accepted |
+| 0066 | [Web pages are HTML the model writes, run in a frame that has no network](0066-web-pages-are-html-the-model-writes-without-network.md) | Proposed |